Clay Minerals (1999)34, 27-37

Use of clay minerals in reconstructing

geological processes" recent advances and
some perspectives
J. S R O D O N

Institute of Geological Sciences PAN, Senacka 1, 31-002 Krak6w, Poland

(Received 15 May 1997; revised 10 November 1997)

ABSTRACT: This article reviews that clay literature from the last ten years, which is devoted to
the applications of clay minerals in the interpretation of geological processes in sedimentary basins.
The results, selected by the author as being of particular interest, are presented, arranged according to
the successive phases of the rock cycle.

The research field defined in the title has expanded WEATHERING

significantly over the last decade. The number of
articles has at least tripled in the period 1992-96 Studies of weathering throughout the last decade
compared with 1986-91, the trend characterizing were dominated by applied, environmental research.
both publications, Clay Minerals', and Clays' and Nevertheless, important advances in understanding
Clay Minerals'. Current advances are largely due to natural processes were made in this period.
widespread use of radiometric dating, stable
isotopes, electron microscope techniques and basin
Kaolinite
modelling software. The fundamental particle
model of Nadeau et al. (1984) inspired different Studies of kaolinite seem the most advanced.
lines of mineralogical research which will soon find Chivas & Bird (1995) summarized results of stable
geological application. isotopic studies in the southern hemisphere (former
Diagenesis remains the most intensely studied Gondwanaland). In Australia, stable isotope data for
domain of clay geology. Research interests centre kaolinite from weathering profiles ranging in age
on sandstones as opposed to shales, which were from Permian to Recent were correlated with the
more widely studied in the pioneer period of clay continent's migration toward the equator. This
diagenesis. The most complete information was correlation can now be used for approximate
accumulated from the North Sea region, as opposed dating of soils of unknown age and for estimation
to the Gulf Coast, which originally provided classic of palaeoaltitudes of dated soils. Australian data
data on shale diagenesis. were confirmed by data from India and South
The state of knowledge in the field of clay America (Chivas & Bird, 1995). They all document
geology has been summarized in detail in books by widespread cold climate kaolinitic weathering.
Chamley (1989) and Weaver (1989). This article Permian kaolins of Australia are interpreted as
presents recent advances in clay science applied to having formed by glacial meltwaters. The presence
solving geological problems, selected by the author of kaolinitic weathering crusts can no longer be
as being of particular interest. They are arranged interpreted as an indicator of tropical weathering,
according to the successive phases of the rock without stable isotope data. It is suggested that the
cycle. Metamorphic and hydrothermal processes are high-latitude kaolinitic weathering crusts of the
not discussed. northern hemisphere were destroyed by glaciation.

9 1999 The Mineralogical Society

28 J. Srodoh

Mizota & Longstaffe (1996) also inferred a cold of high-charge beidellite or vermiculite in Podzols.
climate origin when studying stable isotopes of The processes of smectite formation were stated to
thick Cretaceous and Oligocene kaolinite deposits occur within a few thousand years. These findings
of Japan. Kaolinitization reactions were studied are useful in the interpretation of the origin of
intensely using transmission electron microscopy smectite in marine sediments (see below).
(TEM) (e.g. alteration of biotite: Ahn & Peacor, As opposed to soils, the composition of smectites
1987; Fordham, 1990). Alteration of kaolinite to in weathering crusts does not show such a clear
halloysite in lateritic profiles was documented in pattern. Detailed studies using analytical electron
great detail (Singh & Gilkes, 1992). Such TEM microscopy (AEM) indicate that under the same
studies may aid genetic interpretation of kaolin weathering conditions, smectites of different
minerals in sediments. chemical composition may crystallize locally in
microenvironments, from different parent minerals
(e.g. Aoudjit et al., 1995). A similar variation in
Arenes
smectite chemistry, reflecting chemical gradients
Sequeira Braga et al. (1990) summarized studies developed during alteration, rather than parent rock
of coarse granitic saprolites ('arenes') of Europe, bulk chemistry, was documented in bentonite
from Portugal to Scandinavia. They proposed the deposits (Christidis & Dunham, 1993).
term 'arenization' as a name for a third major Isotope studies of soil smectites seem less
weathering process, in addition to kaolinization and numerous than those of kaolinite. The power of
smectitization, which can be distinguished by its this approach was demonstrated by Stern et al.
textural and mineralogical characteristics (abundant (1997), who presented a stable isotope study of
coarse-grained primary minerals plus very low clay smectites developed in soils on Himalayan molasse.
content). Arenes of Europe show climatic zonality They found that smectite probably crystallizes
of the weathering products, related to temperature during wet seasons and calcite in dry seasons. A
and not to rainfall: from vermiculite and mixed- parallel shift of the stable isotope compositions of
layer minerals in Scandinavia to kaolinite and smectite and calcite shows major climatic change in
gibbsite in Portugal and Spain. Arenes can be the region (either increased aridity or more marine-
used in palaeoclimatic reconstructions. Their sourced precipitation).
geographic relation to cold climate kaolins, detected Illitization of smectite in weathering environ-
by stable isotope studies, remains unclear. ments remains a big unsolved problem of clay
Soil vermiculite, a characteristic component of geology, and is very important for evaluating the
arenes, was studied in detail as an indicator of acid detrital input into sedimentary basins. Wetting and
rain impact (Bain et al., 1990). They concluded that drying cycles, in particular under basic conditions,
the amount of hydroxy-A1 interlayers is pH were suggested as the possible illitization
dependent. This finding is useful for palaeoclimatic mechanism (Eberl et al., 1986 and literature cited
reconstructions based on palaeosoil studies. therein). So far, the most convincing evidence of
this process actually taking place in nature was
provided by Righi et al. (1995), who detected
Smectite and illite-smectite
progressive illitization in polders of the Atlantic
Wilson (1987) reviewed studies of soil smectite, coast of France, reclaimed from the sea over the
stressing the differences of soil smectites with last 350 years. More evidence was presented by
respect to bentonitic smectites. Soil smectites are Berkgaut et al. (1994) who detected randomly
typically Fe-rich beidellites. Hydroxy-A1 inter- interstratified illite-smectite as a major component
layering is common at low pH. Anomalously high of a Quaternary regolith developed on pyroclastics
basal spacings are sometimes recorded, due to other of basalt composition in the Golan Heights. The
types of interlayering (organic?). Righi et al. (1997) origin of illite-smectite is attributed to alteration
summarized their recent work on smectites from during long dry seasons. Both studies point to
Vertisols (basic pH) and from eluvial E horizons of wetting and drying as the illitization mechanism,
Podzols (highly leached, acidic environments). but both are lacking the ultimate proof, i.e. ruling
They found that in both cases, low-charge beidellite out the aeolian input of illite-smectite (e.g. by K-Ar
is the most stable smectite, forming at the expense dating: older than the stratigraphic ages expected in
of montmorillonite in Vertisols and at the expense case of aeolian contamination).
 Clay minerals in reconstructing geological processes 29

It seems clear that the alteration of illite in the Numerous recent studies characterize Mg-clays in
weathering environment is not a simple reversal of lacustrine sediments, usually confirming earlier
the smectite illitization reaction (Wilson, 1987). In findings. Sepiolite and palygorskite are firmly
particular, ordered mixed-layer minerals are not established as indicators of arid climatic conditions
being encountered as alteration products. A study (sabhka, saline lakes, caliche). These results are
by Elsass et al. (1997) indicated a mechanism summarized in a monograph by Jones & Galen
proceeding on crystal-by-crystal basis and produ- (1988). New findings were presented by Hay et al.
cing very heterogenous expandable material. (1995) who investigated Recent sediments of
Eastern African lakes. They documented 8180
evidence of evaporation controlling kerolite vs.
SEDIMENTATION AND EARLY
stevensite crystallization.
DIAGENESIS IN CONTINENTAL
The phenomenon of mechanical clay infiltration,
ENVIRONMENTS
well recognized in soil profiles, was stated as
The most interesting recent developments in this occurring on a large scale in coarse fluvial deposits
field came from a study of the Recent sediments of under conditions of arid/semi-arid climate, owing to
saline alkaline lakes of Eastern California and the a lowered water table. The SEM criteria for
Eastern African rift zone, and ancient sediments of distinguishing infiltrated clays were proposed
the Mediterranean region (Spain, Turkey, Morocco). (Moraes & Ros, 1990).
These studies, published in English, provided more
detailed analyses than previously available for the
SEDIMENTATION AND EARLY
clay neoformation phenomena described from
DIAGENESIS IN MARINE
French Tertiary basins during the 1950s (see
ENVIRONMENTS
Millot, 1970).
Of particular importance is the study of a 700 m
Green marine clays
core from Searles Lake by Hay et al. (1991).
Clastic sediments of this lake undergo little These clay minerals were the first to be
alteration apart from crystallization of small recognized as clearly authigenic in the marine
amounts of clinoptilolite if the pH of the interstitial environment. Current knowledge of these clays
water does not exceed 8. In the pH range 9-10, up has been summarized in an excellent monograph
to 70% of the sediment is recrystallized into an edited by Odin (1988), based mostly on the studies
authigenic Fe-illite, Mg-smectite, K-feldspar and of Recent sediments. Four distinct sedimentary
analcime assemblage. According to stable isotope facies containing Fe-clays have been characterized:
evidence, playa oxidizing conditions favour preci- verdine, oolitic ironstone, glaucony and celadonite.
pitation of Fe-illite and K-feldspar, and open-water The verdine facies is known only from Recent
reducing conditions promote crystallization of Mg- deposits and is characteristic of tropical, very
smectite. This paragenesis can then be used as shallow (5-60 m) marine waters close to a major
indicator of shallow alkaline lake and playa supply of iron (river mouth or outcrops of volcanic
environments. Deconinck et al. (1988), who rocks). Such a restricted occurrence makes this
studied Fe-illites from the Swiss and French Jura, facies an excellent environmental indicator. With
suggested a similar interpretation for the environ- depth, the verdine facies may evolve into the
ment. The conditions and mechanisms of formation glaucony facies. Mineral components of verdine are
of lacustrine Fe-illite, and its chemical composition, interstratified clays composed of 7 A., 14 A and
are clearly different from that of both glauconite smectitic (swelling) layers, called phyllite V and C
and burial-diagenetic illite. Recently, abundant Fe- by Odin (1988). ~. di-trioctahedral 7 A clay mineral
illite of approximately synsedimentary K-Ar age (odinite) was defined by Bailey (1988). Amouric et
was documented in the European Permo-Triassic, al. (1995) documented an interstratification of Fe-
but interpreted as being of pedogenic origin (Jeans kaolinite and Fe-serpentine by a HRTEM study of
et al., 1994). Some Fe-illites produce stratigraphic similar material.
K-Ar ages (e.g. Deconinck et al., 1997), but The mechanism of formation of verdine clays is
sometimes detrital contamination is present and interpreted as neoformation in a confined environ-
cannot be removed (e.g. Le Puy illite; Clauer & ment (pores and cracks in substratum) at the
Srodofi, unpublished data). sediment-water interface. A silicate precursor is
30 J. Sr~176

not needed and verdine minerals can also crystallize and indicative of submarine alteration of basalt. As
on a carbonate substratum. a neoformed mineral, carrying no detrital signature,
Oolitic ironstones are not known from Recent it offers an excellent opportunity for dating the
sediments and are interpreted as diagenetically basalt alteration events (Odin, 1988).
altered clays similar, but not identical to, the
verdine facies. This interpretation is based on
Palygorskite and smectite
similar mineral composition (7 and 14 A clays).
Recently, Hornibrook & Longstaffe (1996) docu- The authigenic vs. detrital origin of palygorskite
mented a transition of grain-coating and pore-lining and smectite in the Atlantic sediments has been
berthierine into chamosite below 70~ They debated vigorously. Thiry & Jacquin (1993) observed
inferred, from oxygen isotope data, a brackish- a lack of smectitic-type weathering in the Cretaceous
freshwater composition for the original crystal- of Europe and a lack of palygorskite in the
lization environment. Cretaceous of onshore Moroccan basins. They
The glaucony facies is defined by the presence of concluded that smectite in the Atlantic Cretaceous
mineral glauconite, which crystallizes by neoforma- sediments must be authigenic, produced by recrys-
tion on different substrata at the seawater-sediment tallization of detrital silicates, and palygorskite must
interface. It became evident that a silicate precursor be also authigenic, crystallized from sinking brines.
is not necessary. Contemporary glauconitization is Deconinck & Chamley (1995) presented evidence
known from open-sea continental margins at low of three varieties of smectite present in Cretaceous
and moderate latitudes and at a range of depth from sediments: (a) soil-derived Fe-A1 smectites; (b)
60 to 1000 m, most often between 60 and 550 m. A recrystallized smectites, distinguishable by idio-
prerequisite for glauconitization is a porous morphic shapes using electron microscopy
substratum residing for prolonged periods of time (Steinberg et al., 1987), and characteristic of low
at the sediment-water interface. The presence of sedimentation rate; and (c) Cheto-type smectites
glauconite is then an indicator of a sedimentation produced by alteration of pyroclastic input. On the
hiatus, the length of which can be estimated from other hand, in the Gulf Coast bottom sediments
the stage attained by the glaucony grains. deposited at high sedimentation rate, Yeh &
Glauconitization of a substratum characteristic of Eslinger (1986) were unable to detect an authigenic
very shallow depth (e.g. shell bioclasts) is a record clay component by stable isotope techniques.
of marine transgression. Based on particle morphology observed by SEM
Mineralogically, glauconitization is a process and TEM, Pletsch et al. (1996) described two types
analogous to illitization: gradual alteration of of palygorskite in onshore Moroccan basins:
neoformed Fe-rich smectite into Fe-illite similar to authigenic in the Atlas Mountains, and reworked
celadonite, through intermediate mixed-layer in locations close to the present-day shoreline. They
phases. Simultaneously, substratum minerals gradu- concluded that deep-sea Atlantic palygorskites are
ally dissolve. Complete glauconitization, whereby detrital, transported mostly by SW winds from
expandability disappears and K20 increases to 9%, nearshore shallow African basins into the ocean.
requires ~1 Ma. It is estimated that, at a stage of Thus they confirmed palygorskite as an unambig-
7% K20, substratum minerals are totally dissolved, uous indicator of shallow-water saline environments
and such glauconite grains should yield correct and arid climate.
stratigraphic ages (Odin, 1988).
A very detailed study of the glauconitization of
BURIAL DIAGENESIS
nontronite that crystallized in the Galapagos
Spreading Centre mounds has been presented by Burial diagenetic processes are controlled predomi-
Buatier et al. (1993). Transition from the verdine to nantly by the bulk composition of the rock. As a
the glaucony facies was documented by Amouric et result, illite is the most common diagenetic product.
al. (1995). Using HRTEM they discovered the Formation of kaolinite and chlorite as dominant
following alteration sequence: Fe-kaolinite- diagenetic minerals requires special conditions
berthierine~lauconite. (high fluid/rock ratio or uncommon composition
Numerous studies of celadonite in ODP materials of the parent rock). Reactions involving all three
have confirmed that celadonite is chemically minerals were studied extensively in the last decade
different from glauconite (more Si, Mg and K) and used to interpret geological processes.
 Clay minerals in reconstructing geological processes 31

Illite proposed formulae take into account only time-

temperature effects (Velde & Vasseur, 1992) or
M e c h a n i s m o f illitization. The single most include pore-water chemistry (Pytte & Reynolds,
important development in illite studies was the 1989; Huang et al., 1993) and chemical hetero-
interpretation of mixed-layer minerals in terms of geneity of the parent smectite (Wei et al., 1996).
fundamental particles and interparticle diffraction, Critics of the kinetic approach point out that none
initiated by McHardy et al. (1982) and developed of the published equations is capable of modelling
by Nadeau et al. (1984). This new concept implied basins of diverse history (Elliot & Matisoff, 1996).
that the entities which evolve during illitization are In particular, for old basins the modelled degree of
not mixed-layer crystals but fundamental particles, illitization is always highly overestimated. The fit
and an understanding of the mechanism of would become even worse if the recent findings of
illitization should evolve from fundamental particle Small (1994, 1995) were taken into account. He
studies. Indeed, Inoue (1986) stated that funda- found experimentally that carbonate and organic
mental particles grow simultaneously in three anions (oxalate, acetate) buffer aK+/aH + in the mica
dimensions (3D), changing their shape from stability field, and the rate of illitization increases
irregular flakes to laths and hexagonal plates. A by several orders of magnitude. On the other hand,
similar trend was described by Lanson & Champion quite expandable illite-smectite persists even in
(1991). Recently, Srodofl et al. (1997) confirmed a Precambrian sediments if the burial history did not
3D growth pattern. They found that the thickness include an elevated temperature event (Price &
distribution of fundamental particles of illite- McDowell, 1993).
smectite is strictly lognormal and follows a unique An alternative viewpoint is held, e.g. by Pollastro
evolution pattern. This finding permits refinement (1993) and Srodofl (1995), who concluded from
of the well-known Kubler crystallinity index analysing expandability data and thermal histories
measurement into a precise measurement of mean of basins ranging from Precambrian to Quaternary
crystallite thickness (Drits et al., 1997). that the expandability of I-S in shales is controlled
The mechanism of illitization in bentonites was by, and indicative of, the maximum palaeotempera-
identified as the growth of fundamental particles tures, other factors being negligible. Such an
(simultaneous nucleation and growth was excluded) approach was used to interpret K-Ar data,
by K-At dating of classes of fundamental particles producing results consistent with a basin history
separated by ultracentrifugation: thinner particles model (Clauer et al., 1997). It is believed to apply
gave older ages (Clauer et al., 1997). This only to shales, where K supply is not the rate-
technique offers an opportunity to determine the limiting step. Additionally, NH4 becomes available
duration of illitization events, as demonstrated by during shale diagenesis and its level in illite is
timing the duration of thrusting in the Montana indicative of the intensity of organic diagenesis
disturbed belt (Clauer et al., 1997). (Lindgreen, 1994 and literature cited therein).
The lognormal distribution of the thickness of In bentonites, which are K-deficient rocks,
fundamental particles does not support the concept illitization is controlled by the kinetics of K
of Ostwald ripening suggested earlier as an migration from outside the bed, as evidenced by
explanation of the illite crystal growth mechanism %S zonation and K-Ar dates (Altaner et al., 1984;
(Eberl et al., 1988; Inoue et al., 1988). Clauer et al., 1997). As a result, illitization in
Controls o f the illitization reaction and palaeo- bentonites lags behind shales (Sucha et al., 1993).
temperature interpretation. Discussions concerning In sandstones, the degree of illitization may
thermodynamic vs. kinetic control of smectite reflect the temperatures of migrating fluids rather
illitization in shales started with Eberl & Hower than the burial temperatures of the entire basin. An
(1976), and continued during the past decade example of such a relationship was presented by
(review in Essene & Peacor, 1995). It is largely Whitney & Northrop (1987). Analysis of the
agreed that the reaction is kinetically controlled. chemical composition of illite and chlorite coex-
Several kinetic equations have been proposed and isting in sandstones led Jahren & Aagaard (1992) to
used to model the evolution of expandability with conclude that these minerals crystallize and
depth in different basins in order to test computer recrystallize in near equilibrium conditions.
models of basin evolution, in the same way as Dating diagenetic events. Reliable dates are
vitrinite reflectance data are being used. The currently available only from bentonites and some
32 ~ Srodoh

types of sandstones (aeolian, beach), free from hydrothermal systems. Studies of diagenetic mate-
detrital mica contamination. Shales produce mixed rials confirmed an lVA1 increase with temperature
detrital/diagenetic ages even in the finest but did not confirm the unique relationship,
(<0.01 gm) separable fractions (Clauer et aI., indicating that the temperature-composition trend
1997). Attempts to extrapolate diagenetic and is affected by the bulk composition of the system
detrital ages by quantifying these components in (Jahren & Aagaard, 1989, 1992; Hillier & Velde,
shale samples were presented (Mossman, 1991; 1992; Spotl et al., 1994).
Pevear, 1992) but they seem to be of only very According to the reviews of de Caritat et al.
restricted use because quantification is often next to (1993) and Walker (1993), no simple relationship
impossible. exists between chlorite polytype and temperature,
Illite dates were used to interpret ages of various although generally lib is the higher temperature
diagenetic processes. Most often, these are ages of polytype. Spotl et al. (1994) documented a gradual
maximum palaeotemperatures related to burial (e.g. l b - l l b polytype transition in Fe-chlorites from
Elliot et al., 1991; Barnes et al., 1992; Clauer et al., sandstones of the Arkoma Basin, Oklahoma, well
1997). However, K-Ar dates of filamentous illites correlated with changes in crystal morphology from
from the Paris Basin are incompatible with burial small thin plates into bigger and thicker plates.
history and indicate a major heating event of They estimated the lower limit of chlorite lib
Liassic age (Mathieu & Velde, 1989; Mossman et appearance as 150-180~ Very similar data were
al. 1992) and accompanying hot-fluid flow (Platt, presented by Hillier (1994).
1993). A heating event of similar age was detected Problems of chemical and polytype chlorite
by Clauer et al. (1996) in the Rotliegendes of palaeothermometry were explained by Curtis et al.
Alsace and N. Germany and was also related to hot (1985) and Hillier (1994) who proposed diverse
fluid flow along major fault lines. Recently, the origins for chlorite coatings in sandstones.
Liassic heating event, much younger than the According to the latter author, Fe-rich chamosite
maximum burial of the basin, was dated in the comes from a berthierine (serpentine) precursor and
Carboniferous of the Upper Silesia Coal Basin Mg-rich clinochlore, from trioctahedral smectite.
(Banag et al., 1997). Similarly, K-Ar dates of The Mg-chlorites have lower IVA1 content than Fe-
filamentous illites in the Rotliegende and Jurassic chlorites. The polytype of Fe-chlorite evolves with
of the North Sea are interpreted as ages of hot fluid temperature from I to lib, whereas Mg-chlorite
flow and hydrocarbon emplacement (Ziegler et al., crystallizes directly as llb. The model of Curtis et
1994 and literature cited therein). Hay et al. (1988) al. (1985) is even more complicated, including
interpreted their K-Ar data for illite-smectite from direct precipitation of chlorite from solution as a
Ordovician tufts in the Mississippi Valley area in product of several mineral reactions. In any case,
terms of regional flow of basinal brines. these models imply that the chemical composition
The K-At or 4~ dates of illite were also of grain-coating chlorite reflects the sedimentary
used to evaluate the duration of thrusting (Altaner environment. Further refinement of the model was
et al., 1984; Clauer et al., 1997) and age of presented by Hillier et al. (1996).
stylolitization (Thomas et al., 1993). Diagenetic evolution of chlorite is complicated
by interstratification. Interstratification with smec-
tire and/or vermiculite was recognized in the 1960s.
Chlorite
It differs from the pattern of illite-smectite
Studies of chlorite, in particular of authigenic interstratification through the exceptional stability
chlorites in sandstones, have expanded rapidly in the of the regular l:l phase corrensite. According to
past decade. Attempts were made to apply chlorite as data summarized by Hillier (1993), estimates of the
a palaeothermometer, utilizing the evolution of its temperatures of appearance of corrensite range
chemistry, polytype, interlayering and morphology between 60 and 160~ Corrensite is then a late
during diagenesis and metamorphism. Reviews of diagenetic mineral, unknown from surface environ-
this literature were presented recently by de Caritat et ments. The transition of corrensite to chlorite was
al. (1993) and Walker (1993). documented in detail from TEM studies by Jiang &
Cathelineau (1988) presented a strong linear Peacor (1994).
relationship between tetrahedral A1 in authigenic Reynolds et al. (1992) and Hillier & Velde (1992)
chlorites and present-day temperature for two active recognized interstratification of chlorite with serpen-
 Clay minerals' in reconstructing geological processes 33

tine and proposed XRD techniques for quantification great extent on the progress made by clay
of such mixed-layering. Hillier (1994) presented data mineralogists. New measurable parameters or
indicating that the serpentine component has a smaller size of measurable objects make new
berthierine composition and that it decreases applications possible.
gradually with increasing temperature and disappears Current developments in clay structure model-
entirely at temperatures between 150 and 220~ ling, such as turbostratic and/or rotational disorder
combined with cis- and/or trans-vacant unit-cells
(Drits et al., 1993, 1996; Reynolds, 1994; McCarty
Kaolinite
& Reynolds, 1995; Moore & Reynolds, 1997) and
Most new information on diagenetic kaolinite has clustering of cations in the octahedral sheet
come from studies of reservoir sandstones in the (Dainyak et al., 1992) should stimulate systematic
North Sea (summary in Bjorlykke, 1992). Early studies aimed at correlating these features with the
diagenetic kaolinite results from flushing sandstones conditions of clay formation and alteration. These
with meteoric water flow; thus maximum develop- techniques of studying bulk properties of clays will
ment of kaolinite is indicative of the proximity of be aided by TEM studies of individual clay
shoreline, the continuity of sandstone bed, and the particles, in order to evaluate the dispersion of
sea level changes. In the absence of flushing, the bulk properties. New tools for reconstruction of
kaolinite + feldspar assemblage is stable until geological processes may emerge from such studies.
120-140~ and then reacts forming hairy illite. New horizons for isotope studies will appear
Experiments of Huang et al. (1986) confirmed the when isotope information becomes available from
role of fluid/rock ratio in altering feldspar into individual crystals (like AEM analyses at the
kaolinite or illite. According to Osborne et al. present time) and if a single-clay geothermometer
(1994), early diagenetic kaolinite crystallizes at becomes a reliable tool (Bechtel & Hoernes, 1990;
different temperatures with different habits: vermi- Delgado & Reyes, 1996). Clay-water fractionation
form between 25 and 50~ and blocky between 50 equations have been refined recently (Sheppard &
and 80~ However, morphological changes may be Gilg, 1996). We will observe expansion of isotopic
related to the degree of supersaturation rather than studies in the domain of weathering, in particular
directly to temperature. applied to smectite weathering products, and to the
Ehrenberg et al. (1993) recognized and docu- processes involving evolution of illite, both
mented a kaolinite-dickite transition at ~120~ in neoformation and alteration.
the Triassic and Jurassic of the North Sea. Polytypic The B e r t a u t - W a r r e n - A v e r b a c h technique
transition is reflected in morphology changes from allowing rapid XRD analysis of crystal thickness
vermiform to blocky crystals. McAulay et al. distribution (MudMaster program, Drits et al.,
(1994) estimated the transition temperature as 1998) will be used to trace the evolution of
80-110~ This reaction was documented also by crystal size in the course of geological processes.
Ruiz Cruz & Andreo (1996) from Permo-Triassic It will help in refining quantitative clay mineral
rocks of Spain and by Lanson et al. (1996) from the analysis and clay XRD peak decomposition
Rotliegend of the North Sea. techniques. Perhaps, detrital vs. authigenic fractions
Late diagenetic (telogenetic) kaolinite develops of different clay components of sedimentary rocks
in sandstones flushed by gravity-driven meteoric will become distinguishable by this technique. For
waters after the tectonic inversion of a basin. illite-smectite, it offers the possibility of measuring
Longstaffe (1992) reviewed the abundant stable directly the thickness distribution of illite funda-
isotope data on kaolinite from the Western Canada mental particles (Eberl et al., 1998).
Sedimentary Basin. Baker & Golding (1992) used Hydrothermal experiments, such as those of
5180 values of such telogenetic kaolinite to date the Huang (1990) and Small et al. (1992), may
uplift of an Australian basin by the technique of provide clues for interpreting pore-fluid composi-
Chivas & Bird (1995). tion from clay crystal morphology.
When the mechanism of illitization and its
controls become better understood, the process
PERSPECTIVES
will be modelled properly and this model will be
Development of new techniques for solving integrated into basin-modelling software as an
geological problems using clays depends to a additional constraint. Hopefully, rapidly expanding
34 ~ Srodor~

studies of diagenetic chlorite will succeed in fractionation between oxygen of different sites in
providing another reliable tool for the reconstruc- illite minerals: a potential single-mineral thermo-
tion of different aspects of the history of meter. Contrib. Mineral. Pet. 104, 463-470.
sedimentary basins. The ultimate input of clay Berkgaut V., Singer A. & Stahr K. (1994) Palagonite
reconsidered: Paracrystalline illite-smectites from
science into basin modelling studies will be realized
regoliths on basic pyroclastics. Clays Clay Miner.
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