Review of aragonite and calcite crystal morphogenesis in thermal spring

systems

Brian Jones

PII: S0037-0738(17)30080-5
DOI: doi:10.1016/j.sedgeo.2017.03.012
Reference: SEDGEO 5180

To appear in: Sedimentary Geology

Received date: 3 February 2017

Revised date: 24 March 2017
Accepted date: 25 March 2017

Please cite this article as: Jones, Brian, Review of aragonite and calcite
crystal morphogenesis in thermal spring systems, Sedimentary Geology (2017),
doi:10.1016/j.sedgeo.2017.03.012

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Review of aragonite and calcite crystal morphogenesis in thermal spring systems

Brian Jones
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Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta,
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T6G 2E3, Canada.

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Corresponding author E-mail: Brian.Jones@ualberta.ca

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ABSTRACT

Aragonite and calcite crystals are the fundamental building blocks of calcareous thermal spring

deposits. The diverse array of crystal morphologies found in these deposits, which includes

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monocrystals, mesocrystals, skeletal crystals, dendrites, and spherulites, are commonly

precipitated under far-from-equilibrium conditions. Such crystals form through both abiotic and

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biotic processes. Many crystals develop through non-classical crystal growth models that

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involves the arrangement of nanocrystals in a precisely controlled crystallographic register.

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Calcite crystal morphogenesis has commonly been linked to a “driving force”, which is a

conceptual measure of the distance of the growth conditions from equilibrium conditions.
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Essentially, this scheme indicates that increasing levels of supersaturation and various other

parameters that produce a progressive change from monocrystals and mesocrystals to skeletal
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crystals to crystallographic and non-crystallographic dendrites, to dumbbells, to spherulites.

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Despite the vast amount of information available from laboratory experiments and natural spring
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systems, the precise factors that control the driving force are open to debate. The fact that calcite
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crystal morphogenesis is still poorly understood is largely a reflection of the complexity of the
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factors that influence aragonite and calcite precipitation. Available information indicates that

variations in calcite crystal morphogenesis can be attributed to physical and chemical parameters

of the parent water, the presence of impurities, the addition of organic or inorganic additives to

the water, the rate of crystal growth, and/or the presence of microbes and their associated

biofilms. The problems in trying to relate crystal morphogenesis to specific environmental

parameters arise because it is generally impossible to disentangle the controlling factor(s) from

the vast array of potential parameters that may act alone or in unison with each other.

Key words: Calcite, aragonite, crystal morphogenesis, thermal springs, mesocrsystals

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1. Introduction

Many thermal spring systems throughout the world (e.g., Waring, 1965; Pentecost, 2005),

are characterized by spectacular arrays of precipitates that are formed of amorphous calcium

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carbonate, calcite, and aragonite. The development and appearance of these deposits is, to a

large extent, a reflection of (1) the precipitation of the different CaCO3 polymorphs, and (2) the

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calcite and aragonite crystal morphogenesis. Factors that control precipitation of the CaCO3

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polymorphs are outline in Jones (2017), whereas the calcite and aragonite crystal morphogenesis

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is reviewed herein.

The calcite and aragonite crystals in spring deposits are commonly characterized by bizarre
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morphologies that include single crystals of various morphologies, dendrite crystals, dumbbells,

fans, and spherulites (e.g., Folk et al., 1985; Guo and Riding, 1992; Jones and Renaut, 1995).
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The fact that “…crystallization in natural environments rarely occurs near equilibrium”
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(Fernández-Díaz et al., 1996, p. 482) means that it is commonly difficult to interpret and
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understand the parameters that control calcite and aragonite crystal morphogenesis.
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Interpretation of the unusual crystal morphologies found in spring systems using classical crystal
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growth model, for example, is difficult and commonly yields conclusions that are open to debate.

Given the importance of CaCO3 to materials science and chemical materials, laboratory

experiments have been routinely used to determine the factors that control calcite and aragonite

crystal morphogenesis. Collectively, these experiments have clearly illustrated that aragonite

and calcite precipitation is complex and controlled by many different parameters. Nevertheless,

these experiments have shown that crystal morphologies can be related to the types of hydrogels

used (Kulak et al., 2007; Meldrum and Cölfen, 2008; Song and Cölfen, 2010; Zhou et al., 2010;

Nindiyasari et al., 2015) as well as the organic and inorganic additives that are in the parent
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solution (Sánchez-Pastor et al., 2011; Asenath-Smith et al., 2012; Dorvee et al., 2012;

Nindiyasari et al., 2014). Extracellular proteins and polysaccharides found in the

biomacromolecules, for example, play a major role in the precipitation of most CaCO3

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biominerals (e.g., Lowenstam and Weiner, 1989; Albeck et al., 1996; Belcher et al., 1996; Wang

et al., 2013). Laboratory analyses, like those conducted by materials scientists, provide

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invaluable information that should be integrated with geological interpretations of precipitates

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found in thermal spring systems.

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With specific reference to deposits associated with thermal springs, the main purposes of

this review paper are to: (1) define the terminology that should be applied to aragonite and
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calcite crystals, (2) determine if classical or non-classical crystal growth underpins aragonite and

calcite crystal growth, (3) discuss the factors that control aragonite and calcite crystal
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morphogenesis, (4) determine the information that can be obtained from different crystal
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morphologies, and (5) assess the importance of understanding crystal morphogenesis for
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interpreting other parameters such as stable isotopes. Although this review shows that there is
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considerable information available on the aragonite and calcite crystal morphogenesis in thermal
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spring systems, it also demonstrates that precipitation is controlled by numerous interrelated

parameters that are extremely difficult to disentangle from each other.

2. Crystal terminology

A plethora of terms have been used to define and describe the myriad arrays of CaCO3

crystals found in spring deposits. Further confusion arises when two or more terms have been

applied to crystals with identical morphologies. In other cases, commonly used terms appear to

have never been formally defined and are used under the assumption that everybody knows what

they mean. Given this situation, the terms that have been applied to CaCO3 crystals are
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reviewed, defined, and their validity and applicability assessed. In all cases, the definitions

adopted herein are based on morphological attributes and all are defined independent of genesis.

2.1. Monocrystal

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Although the term monocrystal has long been used to describe a single crystal, no formal

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definition has been used (e.g., Ramseier, 1967; Thattey and Risbud, 1969). Hence, Meldrum and

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Cölfen (2008, p. 4335) defined a monocrystal as a “...crystalline solid in which the crystal lattice

of the entire sample is continuous and unbroken to the edge of the sample, with no grain

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boundaries”. The critical elements of this definition are that the lattice continues unbroken to the
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crystal edges (Zhou and O'Brien, 2008), the packing of the unit cells is continuous, and the

macroscopic morphology is faceted (Imai, 2014).

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2.2. Porous monocrystal

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Zhou and O’Brien (2008) defined a porous monocrystal as a crystal with numerous internal
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pores that range from a few nanometers to micrometers in size. Zhan et al. (2003) and Li and
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Estroff (2007) also used this term to describe crystals that they grew in an agarose hydrogel. The
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term “sponge crystal” has been applied to a monocrystal with “…continuous voids originating

from a series of neighboring vacancies…” (Inumaru, 2006, p. 157). It is, however, probably

easier to treat it as a porous crystal.

2.3. Polycrystal

A polycrystal, also known as a polcrystalline solid, is formed of random aggregates of

numerous grains/crystallites (Zhou and O'Brien, 2008; Imai, 2016).

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2.4. Mesocrystal

Cölfen and Antonietti (2005, p. 5577) defined a mesocrystal as a “…superstructure of

crystalline nanoparticles with external crystal faces on the scale of some hundred nanometers to

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micrometers….”. Meldrum and Cölfen (2008, p. 4343), noting that mesocrystal is an

abbreviation for “mesoscopically structured crystal”, defined it as a colloidal crystal that is

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formed of nanocrystals that are “…aligned in common crystallographic register…. such that the

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mesocrystal scatters X-rays or electrons like a single crystal and shows birefringence properties

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of a single crystal”. Since then, the term has been defined in many papers, but commonly with

subtle differences in the wording (Niederberger and Cölfen, 2006; Xu et al., 2006, 2008a, 2008b;
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Zhou and O'Brien, 2008; Song and Cölfen, 2010; Seto et al., 2012; Zhou and O'Brien, 2012; Kim

et al., 2014; Bergström et al., 2015). Song and Cölfen (2010, p. 1301) noted that the
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nanocrystals were of “…mesoscopic size (1-1000 nm)” and suggested that single crystals have a
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coherence length of > 100 nm whereas the coherence length in mesocrystals is much smaller.
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Imai (2016) also argued that the constituent nanocrystals should be < 1 µm long. Zhou and
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O’Brien (2012, p. 620) suggested that the “Sole criterion for determining whether a material is a
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mesocrystal or not is the unique crystallographically hierarchial structure, not its formation

mechanism”. Kim et al. (2014) issued a note of caution by pointing out that the appearance of

mesocrystals can be misleading because nanocrystals evident on the surfaces may not be

representative of how the crystal actually grew.

2.5. Mosaic crystals

Also referred to as “mosaicism” and “mosaic structure”, French and Koeberl (2010)

defined a mosaic crystal as a “…single uniform crystal that is formed of a large number of

smaller crystal domains (also called “subgrains”) whose crystal lattices are slightly to
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significantly misoriented to each other.” Although Imai (2014, 2016) attributed this term to

Darwin (1922), the term “mosaic” was not used in that paper.

The definition does not carry any genetic connotations and has been used in many other

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contexts, including shocked quartz crystals (Hörz and Quaide, 1973).

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2.6. Composite crystal, aggregate crystal, crystallites, and subcrystals

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In the geological literature, the terms composite or aggregate crystals have been used to

describe crystals formed of smaller units that have been labeled as crystallites or subcrystals

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(e.g., Jones, 1989; Jones and Renaut, 1996b; Jones et al., 2005). These terms, however, do not
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appear to have been formally defined. Wells and Bishop (1955) in describing amphiboles from

pegmatitic diorites on Jersey noted that “Only a small proportion of the amphiboles are
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homogeneous single prisms: some are composite crystals built up of sub-individuals in parallel
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growth.” For calcite crystals, the terms “composite crystal” (Chafetz et al., 1985; Folk et al.,

1985, their p. 352; Given and Wilkinson, 1985, captions to their Figs. 2, 4; Sandberg, 1985,
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caption to his Fig. 11) or “aggregate crystal” (Binkley et al., 1980, caption to their Fig. 2D-F;

Chafetz et al., 1985, caption to their Fig. 6; Taylor and Chafetz, 2004, captions for their Figs. 12-
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14) have been used as descriptors for crystals that are formed of smaller units, typically with the

same external morphology, that have been referred to as “subcrystals” (Sandberg, 1985) or

“crystallites” (Binkley et al., 1980). Taylor and Chafetz (2004, caption for their Fig. 12) implied

that these terms were synonymous when they described an aggregate crystal formed of

“…hundreds of individual subcrystals (crystallites)….”. In other cases, the smaller component

units have simply been described by their morphology (Chafetz et al., 1985; Given and

Wilkinson, 1985). Jones and Renaut (1996a), Jones et al. (2005), Jones and Peng (2014c) used
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the term “composite crystal” for a crystal that was formed of smaller subcrystals (cf., Given and

Wilkinson, 1985; Sandberg, 1985).

With reference to non-geological crystals, Herbstein (2003, p. 303) stated that “Composite

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crystals are formed by the ordered agglutination of crystals of the same or different types: a

presumed requirement is a close resemblance between the structures of the types.” Desiraju

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(2003, p. 466), however, suggested that a better term would be “cocrystal” because they are

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“…two crystals that are joined together”. A “composite crystal” has also been described as one

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that contains “…at least two components, which have different unit cells within the same

crystal” (Coppens et al., 1990, p. 81).

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Jones and Peng (2014a, 2014b, 2016b) abandoned these terms in favour of the terms

mesocrystals and nanocrystals as defined by Meldrum and Cölfen (2008). This definition is
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maintained herein.
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2.7. Dendrite crystals

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Tschernoff (1879 – cited in Smith, 1965) first used the term “dendrite” to describe treelike

crystals (Doherty, 1980). Buckley (1951, his Fig. 1) and Strickland-Constable (1968, p. 287)
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recognized numerous levels of branching in these crystals by using the terms primary branch (or

needle, stem), secondary branches, and tertiary branches. Although these crystals commonly

develop in one plane, three-dimensional forms are also known (Chalmers, 1964). If the spaces

between the branches are filled-in by calcite precipitation at a later stage, they are known as

“filled-in dendrites” (Buckley, 1951, p. 213). Lofgren (1974, his Table 2) also treated a dendritic

crystal as a “…tree-like single crystal” with the “…restriction that all branches of the dendrite be

part of a single crystal”.

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Although Keith and Padden (1964) did not use the term dendrite, they described crystals

that follow crystallographic or noncrystallographic branching patterns. Based on this, Jones and

Renaut (1995) introduced the concept of “crystallographic dendrites” and “noncrystallographic

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dendrites”. In the former, branching patterns follow crystallographic precepts whereas

noncrystallographic dendrites have branching patterns that do not conform to crystallographic

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directions.

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With the progressive documentation of calcite dendrites from many different hot-spring

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systems throughout the world, it is becoming increasingly clear that these complex crystals are

characterized by many different morphologies. Although Jones and Renaut (1995) introduced
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the terms “scandulitic dendrites” and “feather dendrites” as two specific types, this practise of

naming different morphological forms of dendrites has not continued (e.g., Jones and Peng,
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2012). This can only be attempted once there is a better understanding of the full range of
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morphologies associated with dendrite crystals.

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2.8. Skeletal crystals

Lofgren (1974, his Table 2) defined a skeletal crystal as “Generally acicular crystals that
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are incomplete. They often appear hollow in thin section or have irregular outlines that form

during crystal growth.” Later, Alena et al. (1990, p. 539) used the term “sheath crystal” for

hollow columns/prisms. Jones and Renaut (1996b) used the term “skeletal crystal” in accord

with the definition offered by Lofgren (1974).

Gornitz and Schreiber (1981, p. 787) coined the term “skeletal halite cubes (hoppers)” with

the notion that skeletal crystals are “…those which develop branched, tree-like forms or hollow,

stepped depressions…”. Southgate (1982, p. 393) used “skeletal hopper” for crystals with “…up
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to six depressed or stepped crystal faces.” Given that these crystals are not hollow, they do not

conform to the original definition of a skeletal crystal as proposed by Lofgren (1974).

Although Atanassova and Bonev (2006) used the term “skeletal-dendritic crystals” of

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galena, they did not formally define the term. Their Figures 1 and 2, however, show crystals that

are akin to dendrite crystals as opposed to skeletal crystals as defined by Lofgren (1974).

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2.9. Dumbbell crystals

The term “dumbbell crystal”, also known as “wheat sheaf” or “sheaf of wheat” crystals

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(Garcia-Ruiz, 1985; Dominguez Bella and Garcia-Ruiz, 1987; Chekroun et al., 2004) has been
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used largely as a descriptor and there appears to no formal definition of the term. Fouke et al.

(2000, p. 573), however, described a dumbbell crystal as “…dumbbell-shaped aggregates

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composed of parallel needles that spread at their ends into radiating bundles.”
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These crystals, which can be formed of aragonite or calcite, have also been divided into

“fuzzy dumbbells” (Folk, 1993) and “smooth dumbbells” (Buczynski and Chafetz, 1991).
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2.10. Spherulites
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This term has been used as a descriptor of spherical masses that are formed of radiating

needles (e.g., Folk, 1993). Similarly, Gránásy et al. (2005) described a spherulite as spherical

body with a “…densely branched polycrystalline solidification patterns…”. They suggested that

a spherulite could (1) form directly by radially branching from a nucleus, or (2) involve two

stages whereby a “wheat-sheaf” developed first with the spaces between the terminal bulges

being filled-in at a later time.

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3. Classical versus non-classical crystal growth models

The classical crystal growth model, in the simplest sense, involves an atom-by-atom

addition to a nucleus (Geng et al., 2010). Niederberger and Cölfen (2006) argued that the

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primary building blocks like atoms, ions, or molecules initially form clusters that may attain the

size of a crystal nucleus before ion-by-ion attachment takes place and the unit cell replicates and

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a crystal develops. In contrast, non-classical crystal growth is a particle-based reaction system

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(Cölfen and Antonietti, 2005; Geng et al., 2010) that involves the development of mesocrystals

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through “…the arrangement of primary nanoparticles into an iso-oriented crystal via oriented

attachment” (Niederberger and Cölfen, 2006, their Fig. 1). Subsequent fusion of the
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nanoparticles produces a monocrystal (Niederberger and Cölfen, 2006). The notion of non-

classical crystal growth arose from the experimental precipitation of titania (TiO2) by Penn and
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Banfield (1998a, 1998b, 1999). Although Penn and Banfield (1999) acknowledged that classical
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crystal growth takes place, they also described a second mechanism of crystal growth whereby
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solid particles were attached to a crystal surface in a precisely controlled crystallographic

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manner. Penn and Banfield (1998a, p. 969) argued that this “oriented attachment” involved the
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“…spontaneous self-organization of adjacent particles so that they share a common

crystallographic orientation, followed by joining of these particles to a planar interface”. Since

then, many studies have demonstrated that the “oriented attachment mechanism” is common in

the growth of many crystals of various compositions, including the three CaCO3 polymorphs

(Zhang et al., 2010, their Table 1). Song and Cölfen (2010) argued that alignment of the

nanocrystals might be controlled by (1) a structured organic matrix with oriented compartments

that became filled with crystalline matter, or promoted particle alignment, (2) physical fields or
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mutual alignment of crystal faces, (3) epitaxial growth with mineral bridging connecting the

constituent nanocrystals, and/or (4) alignment of the nanocrystals by spiral constraints.

4. Crystal morphologies in thermal spring deposits

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Aragonite and calcite, which are the two main CaCO3 polymorphs found in thermal spring

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deposits, are characterized by many different crystal morphologies (Fig. 1). The diversity of

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aragonite crystals is, however, far less than that associated with the calcite.

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4.1. Aragonite crystal morphologies

Aragonite crystals, which are typically elongate prisms with hexagonal cross-sections, have
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been documented from many springs, including those in the Kenya Rift Valley (Jones and

Renaut, 1996a, their Figs. 5, 6), China (Jones and Peng, 2014a, their Fig. 6; 2014c, their Fig. 4;
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2016b, their Fig. 8), Italy (Guo and Riding, 1992, their Figs. 6-8; Folk, 1994, his Figs. 8-11), and
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Japan (Okumura et al., 2011; Okumura et al., 2012). Cyclic twinning (Fig. 2A) appears to be a
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characteristic trait of many of these aragonite crystals (Jones and Renaut, 1996a, their Fig. 7A-E;
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Jones and Peng, 2014c, their Fig. 6B; 2014a, their Fig. 4A-C; 2016b, their Fig. 8C).
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Aragonite crystals in spring precipitates are commonly arranged in (1) bushes (Fig. 2B), (2)

dumbbells (Fig. 2C), and (3) spherical arrays (Fig. 2D). Although the bushes are characterized

by branching, they do not appear to be true dendrites (Fig. 2B). Each “branch” in the aragonite

bushes found in spring deposits at Jifei (Yunnan Province, China), for example, is a single

crystal that radiates outwards from common nucleation centres (Jones and Peng, 2014a).

4.2. Calcite crystal morphologies

Oaki and Imai (2003, their Fig. 1) broadly divided crystals into “single crystals”

(monocrystals) and “polycrystals”. For convenience, this division is used herein for the purpose
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of describing the morphologically diverse array of calcite crystals found in spring deposits (Fig.

1).

4.2.1. Monocrystals

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Included in this general category are monocrystals, porous monocrystals, mesocrystals,

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mosaic crystals, and skeletal crystals (Figs. 3-5). Although commonly formed of nanocrystals,

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these crystals do not branch.

Mesocrystals have been documented from many spring deposits, including Big Hills

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Spring, Canada (Turner and Jones, 2005, their Figs. 6, 7, 8A), Fall Creek, Canada (Rainey and
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Jones, 2007, their Fig. 3), Clinton, Canada (Jones and Renaut, 2008, their Fig. 9H), Shuzhishi,

China (Jones and Peng, 2012, their Figs. 9G-M, 10), Jifei, China (Jones and Peng, 2014a, their
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Fig. 12I, K), LaXin, China (Jones and Peng, 2014c, their Figs. 4K, L, 5B, C, F), and Shiqiang,
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China (Jones and Peng, 2016b, their Fig. 10). Mesocrystals have also been produced in

experiments that model spring systems (Rogerson et al., 2008, their Fig. 2E; Pedley et al., 2009,
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their Fig. 7C). Water temperature does not appear to be a controlling factor because the above

list of examples includes a range from cold (Big Hills Spring at ~ 7°C) to hot (e.g., LaXin at
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100°C) springs.

Spectacular examples of calcite mesocrystals are common in precipitates from an unnamed

spring at Lýsuhóll, Iceland (Fig. 3) where the water temperature ranges from 20°C at the vent to

16°C on the distal edge of the discharge apron, which is ~ 6 m from the vent. Mesocrystals

found ~ 3 m from the vent are formed of numerous aligned nanocrystals that are each ~ 1.6 x 1.1

x 0.2 µm (Fig. 3A). Each nanocrystal, however, is formed of even smaller units that are of

variable size, ranging from 100 nm x 90 nm x 50 nm to 250 nm x 200 nm x 100 nm (Fig. 3B-E).

Irrespective of size, all of these smaller units have the same alignment and they are evident on all
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faces of the mesocrystal (Fig. 3A-C). In another sample from the same area, the prismatic calcite

crystals (Fig. 3F) are also formed of perfectly aligned nanocrystals (Fig. 3F-I). Many of these

nanocrystals, however, are incompletely formed and commonly appear to be porous (Fig. 3H, I).

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Not all mesocrystals display a perfect external form. Many calcite dodecahedra, which are

characterized by 12 pentagonal faces, are characterized by beveled edges that result from

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incomplete growth of the crystal faces (Fig. 4). In spring deposits in Yunnan Province, China

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(Jones and Peng, 2014c, 2014a, 2016b), these types of crystals are locally common and typically

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are not attached to a substrate (Fig. 4A). Such clusters usually include crystals of various sizes

and variable development (Fig. 4B, C). Thus, some have poorly developed crystal faces and
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edges (Fig. 4B), whereas others have better developed faces and some have sharply defined

crystal edges (Fig. 4C). Irrespective of their morphology, these crystals are formed of
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nanocrystals (Fig. 4B, C). At Jifei, a PVC pipe that transported spring water from one site to
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another became lined with calcite after six months (Jones and Peng, 2014a). Those precipitates
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included dodecahedrons that, like the crystals in the spring deposits, are of variable size and
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development (Fig. 4D, E). The crystal faces, where developed, are smooth and display no
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clearly defined nanocrystals (Fig. 4D). In areas where the crystal faces are not developed,

however, nanocrystals are apparent in the interior (Fig. 4E). Dodecahedrons in spring deposits at

LaXin (Jones and Peng, 2014c) range from almost perfectly formed crystals (Fig. 4F), to crystals

that are clearly formed of nanocrystals, to those formed of nanocrystals but with poorly

developed smooth crystal faces (Fig. 4G-I). In the latter case, the crystal faces appear to start

growth from a central position and then spread laterally (Fig. 4H). During their initial stages of

development, the faces are ovoid with no evidence of sharp crystal edges (Fig. 4I).
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Prismatic trigonal calcite mesocrystals are one of the most common types of mesocrystals

found in spring systems. They are, for example, important components of the “lily-pads” found

along the margins of one of the pools in the Waikite Spring system, New Zealand (Fig. 5A-F)

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where the water temperature is 95-100°C (Jones and Renaut, 1996b) and from an old, inactive

cool-water spring system near Clinton, British Columbia, Canada (Jones and Renaut, 2008). In

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samples from Waikite, the trigonal prismatic calcite crystals are formed of trigonal nanocrystals

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that are aligned in the same crystallographic register (Fig. 5A-F). All of these mesocrystals are

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porous with the size and shape of the internal pores defined by the packing of the nanocrystals

(Fig. 5B, C). Some prisms are skeletal with walls, formed of aligned trigonal nanocrystals, that
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are arranged around the hollow core (Fig. 5D, F). In spring deposits from Clinton, there are

numerous examples of trigonal prismatic crystals formed of trigonal prismatic nanocrystals (Fig.
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5G-I).
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Collectively, these examples demonstrate that rhombic, trigonal, and dodecahedral

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mesocrystals (Fig. 3-5) are common components of many spring deposits. In this context, it is
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important to note that similar crystal morphologies are evident from geographically disparate
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springs that are commonly characterized by vastly different environmental regimes. The fact

that mesocrystals have not been recorded from every spring deposit can probably be attributed to

(1) most precipitates not being examined on the SEM at the high magnifications required for

their recognition, (2) masking of the nanocrystals by fusion, (3) the fact that crystal growth did

not involve the systematic attachment of nanocrystals to the crystal growth surfaces, and/or (4)

miscommunications because of the lack of uniform terminology to describe these complex

crystal forms.

4.2.2. Polycrystals
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The branching crystals in this general category include crystallographic dendrites, non-

crystallographic dendrites, dumbbells, and spherulites (Fig. 1).

Dendritic calcite crystals (Figs. 6, 7) are common components of many spring deposits

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associated with waters of all temperatures throughout the world (Jones et al., 2000; Turner and

Jones, 2005; Jones and Renaut, 2008; Rainey and Jones, 2009). These three-dimensional

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crystallographic and non-crystallographic crystals, which can be up to 12 cm long, are

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morphologically variable and attempts to find dendrite crystals with similar morphologies from

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different springs have been largely futile. At any given locality, however, the morphology and

structures of the calcite dendrites tends to be relatively constant. In old spring deposits near
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Clinton (Jones and Renaut, 2008), the dendrites are built of small crystals that are nested and

stacked so that branches with relatively consistent architecture have developed (Fig. 7A, B).
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SEM imaging shows that most of these crystals are incompletely formed and possibly skeletal
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(Jones and Renaut, 2008, their Fig. 5). Large dendrites found in a riverside exposure at
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Shuzhishi (Rehai geothermal area, Tengchong, China), which are up to 6 cm high and 3 cm in
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diameter (Fig. 7C, D), have a completely different architecture from those at Clinton. Those
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dendrites, which have a consistent architecture throughout the exposure, are bush-like with

numerous levels of branches that all developed through crystal splitting (Fig. 7C, D).

Comparison of these dendrites with those found in other springs like those at Lake Bogoria,

Kenya (Jones and Renaut, 1995), Waikite hot springs, New Zealand (Jones et al., 2000), and

Lýsuhoóll, Iceland (Jones et al., 2005), further underlines the fact that dendrites tend to be

morphological consistent at a given locality but incredibly variable from locality to locality.

Dumbbells (Fig. 8E) and spherulitic arrays (Fig. 1F) are formed of calcite or aragonite

crystals that radiate from a central point. Examples of dumbbells are known from many springs
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including those in the USA (Chafetz et al., 1991, their Fig. 12F) and Italy (Guo and Riding,

1992, their Fig. 6A, B).

5. Crystal morphogenesis

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5.1. Experimental approach

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Fisher and Simons (1926) and McCauley and Roy (1974) were among the first to

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experimentally grow calcite crystals in gels, and Devery and Ehlmann (1981) subsequently used

experimental data to suggest that the crystal form of calcite progressively changed as the Mg

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content increased. Since then, the importance of CaCO3 to materials science and chemical
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materials has triggered numerous experimental studies for examining the parameters that control

crystal morphogenesis (e.g., Meldrum, 2003; Meldrum and Cölfen, 2008; Song et al., 2009; Song
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and Cölfen, 2011; Sand et al., 2012). Collectively, these experiments are characterized by an
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incredible diversity of the substances used and experimental conditions. Gehrke et al. (2005, p.

1317), for example, noted that they include (1) fast stopped flow techniques with crystallization
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taking place within milliseconds, (2) slow gas diffusion methods, and (3) vapor diffusion

methods whereby thermal decomposition of ammonium carbonate allows for the slow generation
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of CO2. Some experiments are inorganic with patterns of crystallization being linked to

additives such as Mg (e.g., Reddy and Nancollas, 1976; Reddy and Wang, 1980; Meldrum and

Hyde, 2001), Li (Meldrum, 2003), or Sr (Reddy and Nancollas, 1976). Other experiments use

organic macromolecules like chitin or collagen (Song and Cölfen, 2011) in an attempt to mimic

the development of biogenic CaCO3. Although solution-based, many experiments also utilize

aragose, silica, or gelatin hydrogels (e.g., Nindiyasari et al., 2015) to model precipitation in

organic templates like those associated with echinoids and other animals.
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The concept of mesocrystals that develop by non-classical crystal growth mechanisms has

been largely underpinned by the desire to understand how various invertebrate animals control

precipitation of their calcareous skeletons (e.g., Zhou et al., 2009; Wang et al., 2013; Bergström

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et al., 2015). These concepts, however, are also applicable to other geological environments,

include spring systems, where biofilms are common and play a significant role in the

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precipitation of CaCO3 (e.g., Jones and Peng, 2014c). In addition, the growth of crystals under

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laboratory conditions has also led to over-arching concepts that try to explain how and why

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different crystal morphologies are related (e.g., Sunagawa, 1981, 1982).

Sunagawa (1981, his Fig. 2; 1982) argued that with increasing supersaturation levels in the
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parent fluid, crystal morphology changes from polygonal to hopper to dendrite to spherulitic.

Oaki and Imai (2003, their Fig. 1), based entirely on laboratory experiments, related a spectrum
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of crystal forms to a “driving force” (Fig. 8) that Imai et al. (2006, their Fig. 1) later related to the
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“distance from equilibrium”. Oaki and Imai (2003) argued that crystal morphology depended on
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various parameters including the density of the gel matrix used in the experiments. Thus, Imai
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(2016, his Fig. 6) specifically related the progressive change in crystal morphologies to the
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density of the gel matrix in which the crystals had been grown (Fig. 8). Oaki and Imai (2003)

and Imai (2016) argued that kinetic parameters controlled monocrystal development, whereas

diffusion was largely responsible for the more complex crystals at the higher end of the spectrum

(Fig. 8). Sunagawa (2005) also suggested that the progressive change from monocrystals to

hopper crystals to dendrites to polycrystalline forms was related to a “driving force” that was

primarily related to supersaturation levels. This scheme was later adopted by Beck and

Andreassen (2010, their Fig. 6).

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Overall, the precipitation of aragonite crystals seems to have received far less attention than

the precipitation of calcite crystals. Zhou et al. (2009, their Figs. 1-5), however, produced

hexagonal aragonite mesocrystals with readily apparent nanocrystals. Although single crystals

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were the most common, these experiments also produced dumbbells (Zhou et al., 2009, their Fig.

5c, d).

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5.2. Field-based approach

Prismatic hexagonal crystals characterized by cyclical twinning (Fig. 2A) seems to the

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most common morphology of aragonite found in thermal spring deposits. Variance in the
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aragonite precipitates arises largely from crystal size and the manner in which the aragonite

crystals are arranged relative to each other. In many spring deposits, small needle-like aragonite
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crystals collectively form bushes (Fig. 2B), fans and dumbbells (Fig. 2C), or spheres (Fig. 2D).
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Although aragonite crystals in thermal spring deposits are commonly < 1 mm long, crystals up to

4 cm long and 4 mm wide are known from some of the springs in the Kenyan Rift Valley (Jones
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and Renaut, 1996a).

Based on their assessment of calcite crystals found in spring deposits in the Kenyan Rift
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Valley, Jones and Renaut (1995, their Fig. 14) argued that crystal morphology was related to a

“driving force” that included parameters such as supersaturation and supercooling. They used

the term “driving force” used because it was impossible to identify the precise factor(s) that had

triggered precipitation and determined crystal morphology. Changes in the driving force

produced a spectrum of crystal morphologies that ranged from skeletal crystals to

crystallographic dendrites to noncrystallographic dendrites to spherulitic crystals (Fig. 1). In the

context of this scheme, the monocrystals, mesocrystals, mosaic crystals, and porous crystals are

at the low end of the spectrum (Fig. 1). The scheme proposed by Jones and Renaut (1995, their
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Fig. 14) includes elements found in the schemes produced by Sunagawa (1981, his Fig. 2) and is

very similar to the sequence of Oaki and Imai (2003, their Fig. 1) and subsequently modified by

Imai (2016, his Fig. 7a).

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The calcite mesocrystals shown in Figures 2-5 are formed of oriented aggregates like the

nanocrystals that are evident in many laboratory-produced mesocrystals (e.g., Cölfen and

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Antonietti, 2005, their Fig. 17; Kulak et al., 2007, their Figs. 2-5; Helbig, 2008, his Fig. 5;

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Meldrum and Cölfen, 2008, their Fig. 68; Song and Cölfen, 2010, their Fig. 2; Zhou et al., 2010,

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their Figs. 7, 8; Imai, 2016, his Fig. 8). Although the nanocrystals in most laboratory-produced

mesocrystals are of relatively uniform size, this is not universally true for natural mesocrystals
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found in thermal spring deposits. The nanocrystals evident in the tabular mesocrystals from

Lýsuhóll, for example, are of uniform morphology but variable size (Fig. 3A-E). Similarly, in
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some of these natural examples, some of the nanocrystals are incompletely formed (Fig. 3H, I).
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Despite these morphological variations, all of the nanocrystals have a common crystallographic
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register. Irrespective of these nuances, it is readily apparent that calcite mesocrystals are
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common in many spring systems and that non-classical crystal growth is operative.
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6. Integration of data from experimental and natural aragonite and calcite crystals

Ideally, it should be possible to gain a better understanding of the processes that control

aragonite and calcite crystal morphogenesis by merging the information derived from laboratory

experiments with that derived from natural crystals. Unfortunately, there are inherent problems

with each approach and it is commonly difficult to merge the two types of data in a meaningful

way. In part, this is due to the inherent problems associated with each method of analysis.
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6.1. Problems with data from experimental approach

When considered in the content of natural spring systems, the following problems arise

with respect to information obtained from laboratory experiments.

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 The number of variables associated with laboratory experiments are much lower than in

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natural systems. Thus, there is no assurance that a variable deemed responsible for a

specific crystal morphology in the laboratory experiment will have the same affect in

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natural systems where the number of variables and inter-relationships between variables are

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much higher.

 Laboratory experiments are typically of short duration (hours to weeks) and therefore
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contrast sharply with the long periods over which most modern springs have been

operative.
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
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Many laboratory experiments are run at room temperature (typically ~ 25°C) and therefore

may not be good models for thermal springs, where water temperature may be up to 100°C.
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 Many laboratory experiments are abiogenic and hence difficult to apply to natural spring
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systems where microbes and biofilms are common.

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6.2. Problems with data from natural spring systems

In modern active springs, critical issues that arise in assessing the factors that control

crystal morphogenesis includes the following.

 In many studies, there has been an inherent assumption that the classical crystal growth

model underpins all precipitation in spring systems. Hence, the possibility that non-

classical crystal growth mechanisms were operative has been ignored.

 Any attempt to link crystal morphogenesis to specific attribute(s) of the spring water

implicitly assumes that the modern spring waters were responsible for their precipitation.
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This may not always be true because short-term temporal changes in the composition of

spring waters are common in many springs. For Dagunguo hot spring in the Tengchong

area (China), for example, the percentage of CO2 in the gases varied from 49.7 to 99.7%

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between 1980 and 2000 (Du et al., 2005, their Table 4). Changes like these, also known in

other springs in Yunnan and Sichuan provinces, can be triggered by earthquakes (Ren et

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al., 2005), hydrothermal explosions (Shangguan et al., 2005), and time variable

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contributions of CO2 from different sources (Du et al., 2005). Although the example of

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Dagunguo involves the CO2 levels, similar changes in other components of the spring

water can also occur over short time periods.

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 In many cases, linking crystal morphogenesis to specific physical (e.g., water T) or

chemical (e.g., Mg content) attributes of the spring water tacitly assumes that the process is
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inorganic. The presence of organic macromolecules or microbial mats in the spring system
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may, however, have a significant impact on the processes that govern precipitation and
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crystal morphogenesis.
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 Given that many aragonite and calcite crystals in modern springs are typically very small
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(commonly < 1 mm), it is virtually impossible to actively monitor the microenvironment

around a growing crystal. This is especially true if the crystals are growing in the biofilms

generated by the microbes, which are typically characterized by microdomians that are < 1

µm long (Peng and Jones, 2013).

8. Discussion

In the broadest sense, precipitation of aragonite and calcite in laboratory experiments and

natural spring systems is a three-phase system involving water, organic molecules, and solids

(e.g., Sand et al., 2012). Each of these end members, however, encompass numerous parameters
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that can influence crystal morphogenesis. Important aspects of the water, for example, include

temperature, pH, dissolved elements (e.g., Ca, Mg, Sr), and associated gases (e.g., CO2).

Likewise, the organic component varies in terms of its density, porosity, composition, and the

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physical and biochemical characteristics. Collectively, this means that the degrees of freedom

associated with these systems is very high.

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The driving force of crystallization can, in its simplest sense, be equated to the degree of

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supersaturation of the fluid with respect to CaCO3 (e.g., Torrent-Burgués, 1994; Ruiz-Agudo et

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al., 2011). The driving force of crystallization is, however, a thermodynamic measure and in

itself, cannot always explain precipitation because kinetic factors (Torrent-Burgués, 1994) and
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other variables such as the Ca2+ to CO32- ratio (Ruiz-Agudo et al., 2011; Van der Weijden and

Van der Weijden, 2014), pH (Ruiz-Agudo et al., 2011), and/or Mg content (Wasylenki et al.,
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2005) can influence CaCO3 precipitation and crystal growth. As yet, it has proven impossible to
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disentangle these parameters so that the prime controller of precipitation and crystal
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morphogenesis can be clearly identified. As a result, the “driving force”, which is a key element
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of the crystal growth schemes defined by Jones and Renaut (1995), Oaki and Imai (2003), Imai
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et al. (2006), Imai and Oaki (2010), and Imai (2014) is a conceptual magnitude that includes all

of the parameters that control CaCO3 precipitation and crystal morphogenesis. The complexity

of the system has been clearly demonstrated by the inorganic experimental precipitation of

aragonite and calcite whereby different crystal morphologies are produced by adjusting

parameters such as temperature and Mg content (e.g., Reddy and Nancollas, 1976; Loste et al.,

2003; Meldrum, 2003; Song and Cölfen, 2011). Similarly, experimental modeling of organic

systems has shown that variations in hydrogel attributes (e.g., porosity) can play a critical role in

crystal morphogenesis (e.g., Oaki and Imai, 2003; Nindiyasari et al., 2014, 2015; Imai, 2016).
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Nindiyasari et al. (2014), for example, experimentally demonstrated that the solid content of the

gelatin hydrogel influenced crystal morphology because it caused changes in the diffusivity that,

in turn, controlled the volume of aqueous solution so that the classic layer-by-layer growth of

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crystals changed to the aggregate mechanism that produced mesocrystals.

Imai et al. (2006, their Fig. 1) argued that crystal growth is fundamentally controlled by the

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“distance from equilibrium”, which they defined as the difference between the growth conditions

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and the equilibrium state. Oaki and Imai (2003), Kulak et al. (2007), and Imai (2014) suggested

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that euhedral crystals form largely by kinetic-controlled reactions in near-equilibrium conditions

whereas dendritic and spherulitic crystal growth takes place in far-from-equilibrium conditions.
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Critically, as the driving force increases, the crystal growth rate becomes increasingly controlled

by diffusion or heat transfer (Imai et al., 2006).

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In the context of natural spring systems, resolution of the factor(s) that control aragonite
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and calcite crystal morphologies must determine the factor(s) that contribute to the driving force,
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or more precisely the distance between the growth conditions and equilibrium conditions (cf.,
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Imai et al., 2006). A critical decision in this respect is whether or not inorganic or organic
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precipitation was operative and specifically if microbes and biofilms were involved. Such an

assessment is not always straightforward in natural systems because the presence of

biofilms/microbes relies largely on physical evidence of their presence. Although commonly

well-preserved in opal-A precipitates found in spring systems (e.g., Oehler and Schopf, 1971;

Francis et al., 1978; Westall et al., 1995; Cady and Farmer, 1996; Jones et al., 1998, 2003;

Renaut et al., 1998; Konhauser et al., 1999), microbes are rarely preserved in aragonite or calcite

precipitated around thermal springs (Jones and Renaut, 1995; Peng and Jones, 2012). Likewise,

extracellular polymeric substances (EPS) are rarely calcified and evidence of their presence in
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older deposits is sparse. Substrates in one spring at Lýsuhóll, Iceland, where calcite crystals are

being precipitated, for example, are covered with thriving biofilms (Fig. 6). Although biofilms

are obvious in the field, samples of the calcite mesocrystals collected from this spring do not

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contain any calcified microbes and isolated strands of EPS offer the only evidence of microbial

involvement (Fig. 3A-C, F). Likewise, no mineralized microbes or EPS are associated with the

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dodecahedrons shown in Figure 4 and the dendrites shown in Figure 7. Thus, even in these

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young, relatively fresh samples, the lack of preserved EPS or mineralized microbes means that

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there is little or no direct evidence of microbial involvement in the calcite/aragonite precipitation.

With the passage of time and diagenesis, any traces of microbes or EPS would be rapidly lost.
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Thus, in older spring deposits the evidence would typically indicate that microbes played a

minimal role in their formation (e.g., Jones et al., 2004). Evidence for the presence of biofilms
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and/or microbes may, however, come from the crystals themselves. Numerous experiments have
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shown, for example, that mesocrystals commonly develop when precipitation takes place in
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hydrogel media (e.g., Niederberger and Cölfen, 2006; Nindiyasari et al., 2014, 2015), which can
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be viewed as the laboratory equivalents of the biofilms found in spring systems.

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One of the most intriguing aspects of CaCO3 crystal morphogenesis is the contrast between

the arrays of aragonite and calcite crystal forms that are commonly associated with spring

deposits. Aragonite crystals display limited morphologically variability, with most being

cyclically twinned hexagonal prisms with pointed termini (Jones and Renaut, 1996a), which have

been found in many geographically disparate hot spring deposits. Variations in the aragonite

precipitates comes from the assembly of these crystals into bushes (Fig. 2B), radiating fans, or

spherulites (Fig. 2C, D). In contrast, calcite is characterized by an amazing array of different

crystal forms that range from rhombic mesocrystals (Fig. 3), to skeletal crystals, to trigonal
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mesocrystals (Fig. 5), to dodecahedrons (Fig. 4), to various types of complex dendrites (Fig. 7).

Although the reason(s) for the contrast in crystal diversity between the two polymorphs is not

known, it may be rooted in the fact that aragonite and calcite belong to two different crystal

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systems.

Some of the most extreme crystal morphologies develop when the crystal growth

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environment is far-from-equilibrium (Figs. 1, 8). This situation commonly arises where there is

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rapid CO2 degassing from spring waters, especially in situations where the spring waters are

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supercharged with CO2 that may have been derived from the mantle, magmatic bodies, and/or

sedimentary carbonates. In this context, two aspects are important, namely (1) CO2 is commonly
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the dominant gas associated with thermal springs (commonly > 90% of total gases, by volume)

in the Tenchong volcanic area (Du et al., 2005, their Table 4) and the Kenya Rift Valley
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(McCall, 1967; Darling et al., 1995), and (2) the CO2 content is known to vary with time; for
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example, in Yunnan and Sichuan provinces of China, the CO2 varies in accord with earthquake
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activity (Ren et al., 2005), hydrothermal explosions (Shangguan et al., 2005), and temporal
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variations related to the CO2 source (Du et al., 2005). Although CO2 degassing has been
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commonly been linked to the CaCO3 polymorph that is precipitated in a given system (Kitano,

1962), relatively little is known about its influence on crystal morphogenesis. Nevertheless,

crystal morphologies indicative of far-from-equilibrium precipitation (Figs. 1, 8) are common in

springs where CO2 degassing is known or inferred to be high based on independent evidence

(e.g., Renaut and Jones, 1997; Jones and Peng, 2012, 2016b). In some spring systems, it appears

that precipitation of dendrites and other complex morphologies is not continuous but possibly the

result of episodic variations in some aspect of the spring water chemistry. As noted previously,
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temporal variations in CO2 emissions from springs are known and is therefore possible that they

may be the cause of such precipitation.

Although dendritic and spherulitic crystal growth has commonly been linked to high

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supersaturation levels, it has also been argued that the addition of impurities may be responsible

for such crystallization (e.g., Buckley, 1951; Saratovin, 1959; Hill and Wanklyn, 1968; Keezer et

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al., 1968; Doherty, 1980). Changes in calcite crystal morphology, for example, have commonly

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been linked to increasing Mg content of the parent fluid (Devery and Ehlmann, 1981; Fernández-

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Díaz et al., 1996). Meldrum and Hyde (2001), based on laboratory experiments, argued that the

addition of Mg resulted in (1) a wider range of crystal morphologies, and (2) a change from
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single crystals to crystallite aggregates. Specifically, they noted a sequence from rhombs to

elongate rhombs to dumbbells to intergrown spheres as the Mg content increased. The relative
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importance of Mg content as opposed to organics or other variables in natural systems is,

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however, difficult to untangle (Meldrum and Hyde, 2001). Although those experiments
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illustrated the role that Mg may play in crystal morphogenesis, it is commonly difficult to
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translate that into natural spring systems. In many thermal springs in the Kenyan Rift Valley
AC

(e.g., Jones and Renaut, 1996a, their Table 1) and in the Yunnan Province of China (e.g., Jones

and Peng, 2015, their Table 1), for example, the Mg content of the spring water is significantly

lower than the Ca content. This must, however, be treated with some caution because there is no

guarantee that the aragonite and calcite in those springs formed from the present-day waters or

from water that had the same composition as the water today.

Irrespective of the details, there is ample evidence from experimental work and the analysis

of thermal spring precipitates throughout the world that non-equilibrium precipitation of

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aragonite and calcite is common. This carries important implications for other analytical

techniques that are commonly used in the characterization and interpretation of these deposits.

The 18O and 13C of the carbonate are commonly used to gain insights into the conditions

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that existed as precipitation of aragonite and/or calcite took place. Use of the 18O for

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calculating the temperature of the parent water, however, assumes that precipitation was in

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isotopic equilibrium with the water, irrespective of the equation that is used (Kele et al., 2015).

Although some studies have suggested that the rate of calcite precipitation may also affect the

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the oxygen fractionation between and calcite and water (Dietzel et al., 2009; Day and Henderson,

2011; Gabitov et al., 2012), other studies have suggested that there is no clear correlation
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between the two parameters (Kele et al., 2015). As yet, potential linkages between

calcite/aragonite isotope values and crystal morphology have not been evaluated. If the crystal
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morphology is characteristic of far-from-equilibrium conditions, then there is the possibility that

any temperature derived from the isotope will be invalid. This, however, can be difficult to
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determine, especially given that the temperature of spring waters ranges from 0 to 100°C. In
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some situations, the calculated temperatures are outside of this range and thus attest to non-
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equilibrium precipitation (e.g., Jones and Peng, 2012). In other examples, however, the

calculated temperature falls within the 0 to 100°C range even though the crystal morphology

indicates non-equilibrium precipitation (e.g., Jones and Peng, 2016b, 2016a).

Data from laboratory experiments and thermal spring deposits clearly demonstrate that

aragonite and calcite crystal morphogenesis is extremely complex because it is controlled by

many different parameters that are commonly interlinked with each other. Although general

schemes like those proposed by Sunagawa (1981, 1982), Jones and Renaut (1995), and Oaki and
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Imai (2003) provide some guidelines regarding crystal morphogenesis, it has so far proved

almost impossible to link specific crystal types with specific environmental parameters.

With our present knowledge of aragonite and calcite crystals morphogenesis it is usually

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possible to provide a general idea of the conditions that led to growth of particular crystal

morphologies. Dendrites, for example, are usually indicative of non-equilibrium conditions.

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Nevertheless, despite the numerous studies of natural spring systems and innumerable laboratory

SC
experiments it remains impossible to precisely define the exact conditions that leads to the

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precipitation of each type of crystal. This situation exists because it has proven impossible to

disentangle the complicated arrays of externally imposed physiochemical parameters that may, in
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many situations, be further influenced by the microbes that thrive in these spring systems. Even

with laboratory experimental systems, which are far less complex than natural spring systems, it
D

is commonly difficult to exactly pinpoint the prime factor that is controlling CaCO3 precipitation
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and crystal morphogenesis. Future resolution of this problem may come from integration of
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information from detailed analyses of natural spring systems and laboratory experiments. In
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each setting, scale is a major problem because much of the precipitation in spring systems is
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controlled by microscale processes that are extremely difficult to monitor with precision. In

addition, the precise roles that microbes play in the CaCO3 precipitation in natural spring

systems also needs precise and careful evaluation.

9. Conclusions

The aragonite and calcite crystals that grow in thermal spring systems commonly develop

through non-classical crystal growth models that are, in some cases, mediated by the microbial

biofilms that thrive in these systems. Although laboratory experiments provide valuable insights

into the parameters that control aragonite and calcite crystal morphogenesis, it is commonly
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difficult to apply those experimental results to natural thermal spring systems. Equally, however,

it is difficult to determine the underlying causes of crystal morphogenesis in natural systems

because of the problems associated with monitoring those systems and then disentangling the

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parameters responsible for the precipitation of the aragonite and calcite crystals. Nevertheless,

the information that is presently available does allow evaluation of these crystalline precipitates

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in terms of “the driving force”, which is a conceptual measure that reflects all of the parameters

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that control precipitation. Many crystals in thermal spring systems are precipitated under non-

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equilibrium conditions, a fact that must be recognized in the interpretation of other analytical

data, including stable isotopes.

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It is readily apparent from this review that much remains to be learnt about aragonite and

calcite crystal morphogenesis in thermal spring systems. Moving forward, the challenge is to
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develop (1) field techniques that will allow detailed in-situ monitoring of these systems at all
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scales, (2) laboratory experiments that are realistic in terms of thermal spring settings so that the
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specific role that each parameter (e.g., water temperature, pH) plays in crystal growth can be
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determined, and (3) a better understanding of the processes that mediate the precipitation of
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aragonite and calcite crystals in the microbial biofilms. Integration of data from all of these

perspectives should provide a clearer understanding of the role(s) that each environmental

parameter plays in aragonite and calcite crystal morphogenesis.

Acknowledgements

This research was made possible by funding from the Natural Sciences and Engineering

Research Council of Canada. I am greatly indebted to Dr. Robin Renaut and Dr. Xiaotong Peng

who gave me permission to use some of the data and images derived from samples obtained

during joint fieldwork projects over the past 20 years. I am also indebted to George Braybrook
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who took most of the SEM images used in this paper. This manuscript benefited greatly from

the critical reviews that were provided by one anonymous journal reviewer, Dr. B. Wilkinson,

Dr. Ana Alonso-Zarza, and the journal Editor Dr. Jasper Knight.

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Zhou, L.P., O'Brien, P., 2012. Mesocrystals – properties and applications. The Journal of

Physical Chemistry Letters 3, 620-628.

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FIGURE CAPTIONS

Fig. 1. Relationship between crystal morphology and driving force (see text for explanation).

Adapted from Jones and Renaut (1995, their Fig. 14) with monocrystals added herein. P =

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pore; NC = nanocrystal; arrows on nanocrystals indicate growth axis.

Fig. 2. Examples of aragonite crystals from spring deposits found at (A) LaXin (see Jones and

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Peng, 2014c, for detailed information), (B) Jifei (see Jones and Peng, 2014a, for detailed

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information), and (C, D) Eryuan hot springs (see Peng and Jones, 2013, for detailed

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information), Yunnan Province, China. (A) Group of hexagonal aragonite crystals, with

each crystal face having a zig-zag suture line (arrows) that is indicative of cyclic twinning.
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(B) Bushes formed of nested splays of radiating aragonite crystals. This is not a dendrite

because each branch is a separate cyclically twinned crystal. (C) Dumbbell formed of
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aragonite crystals. (D) Spherulitic growth of aragonite crystals.

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Fig. 3. Calcite mesocrystals from unnamed spring at Lýsuhóll, Iceland. Sample ~ 6 m from
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spring vent, water temperature of 16°C. (A) Large, mesocrystal formed of numerous thin,
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rhombic nanocrystals that all have a common orientation. Note strands of EPS (arrow)
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spanning some of the larger gaps. (B) Enlarged view of nanocrystals that form the large

mesocrystals shown in panel A. Each nanocrystal is formed of even smaller nanocrsytals.

Note strands of EPS (arrows). (C-E) Individual nanocrystals, each formed of smaller

nanocrystals, with consistent morphologies despite varying in size. Note associated EPS

(arrows). (F) Group of prismatic calcite crystals and associated EPS (arrows). White letter

G indicates position of panel G. (G) Face of prismatic crystal (from panel F) formed of

stacked nanocrystals with common crystallographic orientations. (H, I) Enlarged views of

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nanocrystals from area shown in panel G. Note consistent orientation of the nanocrystals

even though some are incompletely developed.

Fig. 4. Dodecahedral mesocrystals from spring deposits on cliff face (A-C) and lining PVC pipe

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(D-F) at Jifei (see Jones and Peng, 2014a, for detailed information) and Gongxiaoshe,

LaXin (see Jones and Peng, 2014c, for detailed information), Yunnan Province, China. (A)

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Unattached dodecahedral crystals of various sizes and shapes associated with filamentous

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microbes and ESP. (B, C) Incompletely formed dodecahedral crystals with poorly formed

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smooth crystal faces, missing or poorly developed crystal edges, and interiors formed of

nanocrystals. (D) Incompletely formed, unattached dodecahedron crystal from precipitates

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lining interior of PVC pipe that formed within 6 months. Note variable development of

crystal faces, edges, and nanocrystals. (E) Corner of dodecahedral crystal, from PVC pipe,
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showing smooth, poorly formed crystal faces, lack of crystal edges, and interior formed of
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nanocrystals. (F) Almost complete dodecahedral calcite crystal from PVC pipe. (G) Two
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interlocking, unattached and incompletely formed dodecahedrons with variable

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development of crystal faces and interiors formed of nanocrystals. (H) Dodecahedron

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mesocrystal with poorly developed crystal faces, no crystal edges, and interior formed of

nanocrystals. (I) Enlarged view of lower left corner of crystal shown in panel H, showing

nature of crystal faces and interior of crystal.

Fig. 5. Examples of trigonal mesocrystals from (A-F) Waikite Spring in New Zealand (see Jones

and Renaut, 1996b, for detailed information) and (G-I) Clinton spring, British Columbia,

Canada (see Jones and Renaut, 2008, for detailed information). (A) Side of trigonal prism

showing constituent nanocrystals. Note hollow core. (B, C) Views down c-axes of trigonal

mesocrystals showing trigonal outlines of the constituent nanocrystals. (D) Skeletal trigonal
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crystal with walls formed of trigonal nanocrystals. (E) Enlarged view of upper left corner

of trigonal crystal shown in panel D. Note common orientations of constituent

nanocrystals. (F) View down wall of trigonal mesocrystals showing layer formed of

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trigonal nanocrystals with common crystallographic orientations. (G) Group of trigonal

nanocrystals with common orientation. (H, I) Views down c-axes of trigonal mesocrystals

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showing constituent nanocrystals.

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Fig. 6. Scanning electron microscope photomicrographs showing general attributes of calcite

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dendrite crystals from (A, B) Clinton, Canada (see Jones and Renaut, 2008 for detailed

information); (C, D) Tengchong, Yunnan Province, China (see Jones and Peng, 2012 for
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detailed information); and (E) Eryuan, Yunnan Province, China (see Peng and Jones, 2013

for detailed information). (A) Cross-section through large dendrite crystal showing multiple
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levels of branching. Box labeled B indicates area shown in panel B. (B) Branches formed
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of stacked calcite crystals. (C, D) Complex calcite crystals with new branches developing
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through crystal splitting. (E) Calcite dendrite with branches developing from main branches
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(arrows).
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Fig. 7. Dendrites associated with modern spring at Lýsuhóll, Iceland. (A) Example of three-

dimensional calcite dendrites growing in shallow pool on outflow apron of unnamed spring

at Lysuholl. Although not evident in the photograph, the dendrites are coated with a thin

layer of EPS. (B) Area of outflow apron, close to area shown in panel A, but with calcite

precipitates largely obscured by actively growing microbial mat formed of filaments and

EPS.

Fig. 8. Diagram showing relationships between crystal morphology, driving force, and rate

determining processes. Main part from Oaki and Imai (2003, their Fig. 1) is based on
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experimental laboratory precipitation of various types of crystals in different types of gels.

The relationship between the density of gel matrix and crystal form is from Imai (2016, his

Fig. 7).

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Figure 8
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