LOW TEMPERATURE HYDROTHERMAL SYNTHESIS FROM

KAOLINITE
Review jurnal : LOW TEMPERATURE HYDROTHERMAL SYNTHESIS FROM DOLOMITE OR CALCITE,
QUARTZ AND KAOLINITE by P. BAYLISS and A. A. LEVINSON, Department of Geology, The University of Calgary, Alberta,
Canada

TKG 739 Geochemistry of Ore Deposits

Lecture : Dr. Agung Harijoko

Name :

Andi Faesal

GEOLOGICAL ENGINEERING DEPARTMENT

FACULTY OF ENGINEERING
GADJAH MADA UNIVERSITY
2014/2015

INTRODUCTION
MONTMORILLONITES have been synthesized from mixtures of glasses, gels and
pure crystalline chemicals as summarized by Deer et al. (1962). Kinetic experiments on the
crystallization of amorphous silica by Carr and Fyfe (1958), and Campbell and Fyfe (1960)
indicate that amorphous reactants in laboratory experiments may not duplicate natural
processes. Some experimentors, for example Hawkins (1969), alter natural glasses and rocks
hydrothermally, but they introduce a fixed starting material of composition known only
approximately. Naturally occurring minerals (various carbonates, quartz, and kaolinite or
illite or feldspar) have been used to synthesize montmorillonites by Coombs (1960),
Levinson and Vian (1966) and Levinson and Day (1968).
The work reported in this paper is an extension of the previously published
hydrothermal synthesis work, but under more controlled conditions. In this study, 140 bomb
runs were made with various quantities of hand picked (purity confirmed by X-ray
diffraction) naturally occurring dolomite or calcite, quartz and kaolinite (1 Md). Reactions of
these minerals from stoichiometric quantities approached completion in contrast to earlier
experiments in the literature, which show partial reaction from nonstoichiometric amounts.
React: ions in kaolinite systems in nature help explain the results of diagenesis, low-grade
metamorphism and hydrothermal activity. Temperatures of about 2500C are commonly found
at depths of 20,000 to 30,000ft in sedimentary areas with normal thermal gradients.
EXPERIMENTAL
Approximately 0.4g of a <200 mesh mixture together with 50 ml of distilled water
was placed in a stainless steel bomb with 100-150 ml capacity. The quantity of water
vaporized under these experimental conditions is calculated to be less than 2 ml at all
temperatures. The water generates a vapour pressure of 40 bars at 250 0C or 90 bars at 3000C
Smaller quantities of starting material resulted in difficulties owing to a low yield and the
excessive differential loss of the elements into the aqueous phase. Larger quantities of
material or less water increased the reaction time because the surface area to water ratio has
decreased. Too much water (more than 50 ml) does not allow space for the 30 per cent
increase in volume at 3000C and also makes shaking the solids into suspension more difficult.
With the 0.4g of material used, experience indicated that at least 6 weeks reaction
time is needed at 2500C and three weeks at 3000C for the mass transfer to approach
equilibrium. Each bomb was shaken every week to increase the reaction rate, and air

quenched when removed from the oven. The products, both solids and water, were measured
to check for leakages in the bombs. Occasionally the pH and magnesium and calcium content
were measured in the product waters. The solids (powder) were identified after glycol
treatment with the X-ray diffractometer. Accurate measurement of the calcite d1024 was used
to calculate the percentage of magnesium in the newly-formed calcite with the aid of the data
by Graf (1961, p. 1297). The composition of the montmorillonite was calculated from
chemical analysis for Si, AI, Mg and Ca determined by atomic absorption, after the calcite
had been completely removed with two cold 1% acetic acid (pH 5,5 treatments and
confirmed by X-ray diffraction. This treatment has minimal effect on other minerals as found
by Chester and Hughes (1967).
DISCUSSION
The reactions described in Table ! are controlled by the fugacities of CO 2 and H20
with the Ca2+, Mg2+, Al3+ and Si4+ conserved among the solid phases. These reactions are
hypothetical in the sense that some of the starting elements remain in solution rather than
entering one of the products.
Table 1. Theoretical chemical equations based on hydrothermal synthesis

The formation of talc (equation 1) is limited, since Mg-trioctahedral montmorillonite

forms rather than talc when either the solids to water ratio is low (4g/l.) or the gas pressure is
low (Pcoz= 1 bar). Calculations show the approximate activities in these experiments are log
(aMg2+/ aH2+) = 7 and log(aca2+/ aH2+)= 8. These experimental results agree with the theoretical
activity diagram (p. 135) of Helgeson et al. (1969), which show talc and calcite are stable at
these temperatures providing that the Pcoz is sufficiently high. Their theoretical equilibrium
diagrams which are based on mass transfer calculations assume that the reaction products

maintain equilibrium with the aqueous phase. This assumption appears realistic for both
experimental and natural processes as demonstrated by the kinetic work of Helgeson (1971).
Equation (1) has been studied extensively by Gordon and Greenwood (1970) and Metz and
Puhan (1970) at higher temperatures and pressures. Only a small quantity of aluminum
prevents the formation of talc (equation 2). The rare occurrence of talc in sediments is
partially explained by this reaction, since aluminum-bearing minerals such as kaolinite or
illite are ubiquitous.

Fig. I. Dolomite, quartz and kaolinite (equations 1, 2 and 3) react in ahydrous environment at 300 0C to form products
illustrated in the ternary diagrams. The area within the dotted line represents montmorillonite and calcite. Each circle
represents a single bomb reaction.

Fig. 2. Dolomite, quartz and kaolinite (equations 1, 2 and 7) react in a hydrous environment at 250 0C to form products
illustrated in the ternary diagram. The area within the dotted line represents montmorillonite and calcite. Each circle
represents a single bomb reaction

Fig. 3. Calcite, quartz and kaolinite (equations 4 and 5) react in a hydrous environment at 300 0C to form products illustrated
in the ternary diagram. Each circle represents a single bomb reaction.

Fig. 4. Calcite, quartz and kaolinite (equations 4-6) react in a hydrous environment at 250 0C to form products illustrated in
the ternary diagram. Each circle represents a single bomb reaction.

The Ca/(Ca+ Mg) ratio in the water recovered from the bomb with dolomite, quartz
and kaolinite (equations (1) and 2) is approximately constant at 0.9 with calcite formation
acting as a buffer. Therefore since the Mg is precipitated preferentially, the Mg-rich minerals
such as talc and Mgtrioctahedral montmorillonite are formed in these experiments. The

MgCO3 content of calcite formed in these reactions was 1 per cent at 250 0C and 2 per cent at
3000C These values of the solid substitution range of MgCO 3 for CaCO3 in calcite at low
temperatures reasonably extend the high temperature data of Graf (1961). The Mg
trioctahedral montmorillonite has a composition range because of the substitution of 2Al 3+ for
Mg2++Si4+ and also a small cation deficiency in the octahedral positions. Its approximate
compositional range is shown diagramatically in Fig. 1, and Fig. 2 illustrates that the
aluminum substitution is limited at lower temperatures. The compositional range of the Mg
trioctahedral montmorillonite (equation 2) in Fig. 1 is significantly different from the
dioctahedral montmorillonite (equation 4) in Fig. 3. Failure to synthesize a solid solution
series between these minerals accords with a corresponding hiatus in the naturally occurring
minerals.
Both the metastable hexagonal and stable triclinic anorthite polymorphs are formed
(equation 5) at 2500C and above, but if dolomite is substituted for calcite in the starting
material, then anorthite only formed above 2880C (equation 3). The metastable hexagonal
anorthite is formed in the presence of more water (40ml average), whereas the stable triclinic
anorthite formed in the presence of less water (20 ml average). These data do not agree with
those of Goldsmith and Ehlers (1952), who considered the role of water as principally that of
a flux with no systematic etfect on either compound. Anorthite has a stability region above
3300C at 500bars according to the reversible reaction of wairakite = anorthite + quartz +
water (Liou, 1970).
The Ca-zeolite garronite (equation 6) which formed at 2500C but not at 3000C was identified
by comparison with X-ray data of Taylor and Roy (1964) who also indicate that garronite
converts to wairakite at 2950C If dolomite is substituted for calcite in the starting material
then garronite formed together with montmorillonite (equation 7). Compositions are not
given because of the variability of both montmorillonite and garronite. The rarity of garronite
in nature suggests that it is metastable; however if garronite has a stability field relative to
wairakite, then the data of Liou (1970) indicate it will be at a much lower temperature than
3300C Its metastability is also implied by its unexpected absence in most bomb runs (Figs. 2
and 4).
Some other minerals which could theoretically form from these compositions are chlorite
Mg5Al (Si3Al)O10(OH)2 and tremolite Mg5Ca2Si8O22(OH)2. Stoichiometric amounts of
dolomite or calcite, quartz and kaolinite were heated to attempt to synthesize the above

minerals,

but

they

were

not

observed

to

form

under

these

experimental

conditions.Calculations, based on solubility and ionization constants of calcite or dolomite in

aqueous solutions under these experimental conditions in Clark (1966), show Pco2 to be 0001 bars with almost no dissociation of CO2. However CO2 is a reaction product in all the
equations to produce Pco, up to 3 bars calculated from the decomposition of the starting
carbonate of either dolomite or calcite. The diagrams (pp. 125, 155, 158 and 165) of
Helgeson et al. (1969) indicate Pco2 prevents the synthesis of chlorite and tremolite.
The pH of the water recovered from the bombs was approximately neutral (6.0-7.2) so the pH
in the bomb at 300~ would be 5 to 6 (Clark, 1966). Therefore these experiments at low
temperature and pressure with slightly acid pH which approximate natural conditions are
applicable to diagenesis, low grade metamorphism and hydrothermal alteration. The reactions
approach completion, which indicates that the products are more stable than the reactants in
this environment however, no exact conditions of temperature and pressure were established
by reversible reactions.
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