MORPHOLOGY OF KAOLINITE CRYSTALS SYNTHESIZED UNDER ... 43/43-3-353.pdf · more elongated, curved and irregular outlines when gels with Si/AI ~ 1 are hydrotherrnally treated. Key Words--Crystal

Clays and Clay Minerals, Vol. 43, No. 3, 353-360, 1995.

MORPHOLOGY OF KAOLINITE CRYSTALS SYNTHESIZED UNDER HYDROTHERMAL CONDITIONS

SAVERIO FIORE, 1 F. JAVIER HUERTAS, 2 FRANCISCO HUERTAS, 2 AND Josf/LINARES 2

Istituto di Ricerca suUe Argille, CNR, P.O. Box 27, 85050 Tito (PZ), Italy

2 Estaci6n Experimental del Zaidin, CSIC Profesor Albareda, 1, 18008 Granada, Spain

Abstract--Scanning electron microscopy has revealed the presence of spherical, lath and platy kaolinite in gels with SL/A1 atomic ratio ranging from 1.84 to 0.76 that are hydrothermally treated under different temperature and time conditions. Hemispherical structures and excavated zones, at different stages of evolution, have been observed on the surface of the gel grains, indicating that spherical particles do not precipitate from the solution but are generated from the gels. The quantity of spherical particles depends on temperature, time and the chemical composition of the starting gel. Products from starting material with Si/A1 ~ 1 yield the highest quantity of these particles. Being metastable, sphere dissolution controls the chemistry of the solution and consequently the morphology of the precipitating crystals thus producing more elongated, curved and irregular outlines when gels with Si/AI ~ 1 are hydrotherrnally treated. Key Words--Crystal growth, Electron microscopy, Kaolinite, Laths, Morphology, Plates, Spherules, Syn- thesis.

I N T R O D U C T I O N

A considerable body of literature is devoted to the experimental crystallization of kaolinite because of its frequency in geological environments and also because of its use in many industrial fields. Consequently, as stated in a detailed review by Van Oosterwyck-Gas- tuche and La Iglesia (1978), the problem of the genesis of the mineral has been examined from many points of view. As it is expected, different physico-chemical conditions of the medium give rise to different morphological and textural appearances. Detailed direct observations by scanning electron microscopy by Kel- ler (1976a, 1976b) showed that kaolinite may exhibit distinct size and morphology depending on its origin. Furthermore, throughout the last decade, research has contributed greatly to the morphological description of the mineral. In particular, besides the well known hexagonal outline, kaolinite of spherical morphology has been reported. This was synthesized for the first time by Tomura et al (1983) starting from non-crystalline aluminosilicate gels under hydrothermal conditions. The presence of spherical particles associated with elongated crystals and hexagonal plates, however, was previously reported in experiments of synthesis from gels under hydrothermal condition (Rayner 1962, De Kimpe et al 1964, Rodrique et al 1972), but they were not identified as kaolinite. The formation of such a peculiar morphology cannot be regarded as an un- usual occurrence, because it has been reported several times (e.g., Tomura et a11985a, 1985b, 1993, Huertas 1991, Huertas et al 1993a). Regarding the mechanism of growth of the mineral, it was suggested (Tomura et al 1985b) that spherical particles precipitate from solution with a high degree of supersaturation at the be-

Copyright �9 1995, The Clay Minerals Society

ginning of the synthesis process, whereas platy kaolinite grows at relatively low degrees of supersaturation. As far as we know, conditions governing the growth of lath kaolinite in a hydrothermal system have not been fully discussed.

The possibility that spherical and platy kaolinite growth occurs at different times is supported by results of a kinetic study. Huertas (1991) considered that the process of crystallization of kaolinite may not be a single step process because the expected quantity of kaolinite production based on theoretical calculations was higher than the real experimental values obtained. However, a two stage kinetic model (Huertas et al 1993b) is consistent with both experimental data and theory.

It is not clear whether a relationship between spherical, lath and platy kaolinite and mechanisms of growth can be established or not. In our opinion, a possible way to contribute to the question is to embark upon a meticulous observation of synthetic products. In the present paper, the results of microscopic research car- ded out on several dozens of samples are reported and possible explanation for the mechanism of growth of the mineral have been proposed.

MATERIALS AND METHODS

Kaolinite samples were obtained, under hydrothermal conditions, starting from non-crystalline aluminosilicate gels. Details about the synthesis method are given in Huertas et al (1993a) and are reported in Table 1. Six sets of runs were carried out using six gel com- positions and 176 samples were synthesized.

The products of the runs from gels with Si/A1 from 1.84 to 0.99 were already characterized by Huertas

353

354 Fiore et al Clays and Clay Minerals

Table 1. Synthesis condition.

Gel composition (Si/A1) Solution Solid/solution Temperature (*C) Pressure Time (days)

1.84, 1.56, 1.26, 0.99, 0.84, 0.76 0.1 M KOH 2.5 g/10 ml 150, 175, 200, 225, 250 Equilibrium water pressure 0.5, 1, 2, 4, 8, 15, 30, 60, 120

(1991), using X-ray diffraction, infrared spectroscopy, and thermal analysis, and they contained up to 70 wt. % kaolinite, the only detectable crystalline phase. The products from the other two gels (Si/A1 = 0.84 and 0.76) were characterized for the present research using the same techniques. Kaolinite was present at up to 80 wt. %. It should be remarked that bohemite never occurred, even if gels of a very high aluminium content were treated.

Morphological study was carried out by scanning electron microscopy (SEM, Cambridge Stereoscan $360) coupled with an energy dispersive X-ray spec- trometer (EDS, LINK AN 10000) and by transmission electron microscopy (TEM, Zeiss 10MC). A drop of the material, dispersed in distilled water by an ultra- sonic probe, was placed on the holder and air-dried at room temperature. For SEM studies, a carbon stub was used to avoid interference from the holder during EDS analyses. Samples were coated with a carbon film. For TEM examination, a microgrid covered with a Form- var film stabilized with carbon was used as a holder; the working voltage was 80 kV.

RESULTS

Use of SEM gave an important contribution to un- derstanding the mechanism of formation and evolution of the three morphologies observed in all the products of the synthesis: spheroidal particles, lath or elongated crystals, and hexagonal plates. Relicts o f gel were also detected in the samples.

e - -

Figure 1. Scanning electron micrograph of aggregates of spherical kaolinite. Product synthesized from gel with Si/AI

0.99, at 225~ for 721 hours.

Figure 2. Transmission electron micrograph of "crushed" particles. Product synthesized from gel with Si/A1 = 1.26, at 225~ for 48 hours.

Figure 3. Scanning electron micrograph of clusters of "lath" kaolinite (see text). Product synthesized from gel with Si/A1 = 1.26, at 225~ for 736 hours.

Figure 4. Scanning electron micrograph of rose-shaped aggregates showing the aciculate part of the crystals connected by very thin membranes. Product synthesized from gel with Si/A1 = 1.84, at 225~ for 792 hours.

Vol. 43, No. 3, 1995 Morphology of Synthetic Kaolinite 355

The spheroidal particles had a well defined outline and their dimension was rather constant in all the products of the runs, ranging from 0.25 to 0.35 #m (Figure 1) in diameter. Particles exhibiting a partial spherical morphology, resembling crushed spheres, were ex- tremely rare. Figure 2 shows the best documentable example of such an occurrence observed by TEM. The spheroidal particles were usually associated in groups composed of a very great number of individuals. But it was much more frequent to find them as either isolated spheres or near to lath/platy crystals forming groups with them or around gel grains. Microscopic observations on the spherical particles as a whole indicated the absence of void space. Their chemical composition does not depend on the composition of the starting gel; as showed by EDS analyses, the Si/AI atomic ratio is always near 1.

The lath crystals appeared in large clusters or in small rose-shaped aggregates (Figure 3), which were associated with the other two morphologies without a clear preference. The greater part of elements forming aggregates cannot be defined as lath s e n s u s tr icto; in fact, they have complex shapes composed of a flat central area from which rigid laths depart with no defined geometry. Sometimes the lath portions of such particles were found to be connected by thin delicate membranes, which gave the aggregate the flake-like appearance (Figure 4). Elongated crystals with well defined outlines were also present (Figure 5).

Platy crystals exhibited pseudohexagonal outlines and formed little stacks (Figure 6), of up to 0.05 #m in thickness, or rose-shaped clusters, whose dimensions could reach 5 #m in diameter. TEM investigations also permitted us to identify the common presence of platy crystals with well defined hexagonal outlines (Figure 7).

Before the hydrothermal treatment, the starting material exhibited a morphology resembling a stack of curl- ing leaves or a stretched sponge, although grains having a compact appearance were also present (Figures 8, 9). They were also characterized as being smaller in size and higher in aluminium content. With regard to this,

---'4

Figure 5. Transmission electron micrograph of elongate crystals of kaolinite. Product synthesized from gel with Si/A1 = 1.84, at 225"C for 48 hours.

Figure 6. Scanning electron micrograph of stacks of platy kaolinite. Product synthesized from gel with Si/A1 = 0.84, at 200"C for 192 hours.

Figure 7. Transmission electron micrograph of pseudohexagonal crystals of kaolinite with well defined outlines. Product synthesized from gel with Si/A1 = 1.26, at 150~ for 763 hours.

Figure 8. Scanning electron micrograph of gel grains with compact texture. Si/Ai = 1.84.


it can be remarked that the chemical composition o f grains from the same gel may change considerably, as can be seen in Figure 9, where grains having Si/AI ratio from 1:1 up to 10:3 are pointed out. The richer in silica the gels are, the more compact the grains are.

As previously stated, the gel had not been completely converted into kaolinite and the microscopic observation produced surprising results. First of all, the grains became less spongy and as the temperature o f the runs increased they became more and more compact. Sec- ondly, their surface was not planar but pimply, due to the presence of many hemispherical structures which were integral parts of the gel (Figure 10). It can be seen that there are analogies with the swelling humps documented in non-crystalline Fe-Si-Al-oxyhydroxides (Eggleton 1987) and in volcanic glass (Eggleton and Keller 1982, Tazaki et al 1992, Fiore 1993).

The abundance o f the three morphological types depends on growing conditions. Several at tempts to quantify them by image analysis were carried out, but the results were not encouraging. The ratio between spheres, laths and plates, est imated in different micro- graphs of the same product may produce different re- suits. However, it was possible to make a rough esti- mat ion and the "evolut ion" o f the different popula- tions was detected in the products from the runs. The relationship between the three parameters - - temperature, t ime and gel compos i t ion- -and the morphologies may be summarized as follows.

Temperature

Temperature had a direct influence on the abundance of the three shapes of kaolinite occurring in the synthesis products. At low temperatures, the gels were only partially transformed, with the leafy structure still remaining, but they were found to be more compact in comparison with the initial products. Surfaces frequently showed hemispherical humps or were covered by spheres. As temperature increased, the amount of laths and plates increased and rosette aggregates and isolated hexagonal plates also appeared. I t was not uncommon to find spheres, rose-shaped aggregates and plates interrelated with each other. As temperature increased the number o f spheres decreased and their size did not change appreciably.

~ m e

As the aUocation of t ime was increased, crystals formed at a given temperature grew and became less fibrous, their arrangement became more orderly and the amount of laths and plates increased. At first, the populat ion of spherical particles increased but later decreased; the rate o f such a t ransformation seemed to depend directly on temperature and therefore at higher temperatures the same number of particles were formed in less time.

Figure 9. Scanning electron micrograph of a gel (Si/AI = 1.54) showing grains with distinct Si/AI ratios.

Gel composition

The Si/AI ratio o f the starting material exerted a control on both the abundance of the three morphologies and on their shape. Hexagonal plates were predominant when Si/AI in the products from the gels are richer in A1 or richer in Si. Otherwise, lath crystals are predominant in the product with Si/AI near 1. With regard to spherical particles, it appeared that products having a Si/A1 near 1 underwent a more extensive change than Si-richer and Al-r icher gels, since a greater number of particles were formed. Regarding the lath crystals, they exhibited a tendency to bend and to form more disordered aggregations. It seems as though there were a great number of crystallization nuclei producing a massive and disordered growth.

DISCUSSION

Microscopic observations carried out in the present study suggested that spherical particles of kaolinite may grow directly from these gels. They started to grow as small vesicles, subsequently their size increased until they detached from the surface. This would explain why spherical kaolinite is not formed under hydrothermal condit ions using amorphous calcium silicate and a luminum chloride (Tomura et al 1993) and why spheres frequently occurred around gels, as was also reported by previous authors (e.g., Rodrique et al 1972, Tomura et al 1983, 1985b). An obvious question then arises: How are spheres formed from the gel? It is l ikely that the growth of such structures cannot be seen as a mechanism of exsolution from a medium with a relatively high density (the gel) immersed into another of a lower density (the solution), as a consequence o f a rise in temperature. In fact, spherical particles always have a Si/A1 ratio close to 1, irrespective of the chemical composit ion of the starting gel, while an exsolution mechanism may give origin to products with a distinct Si/A1 ratio depending on gel composit ion. Likewise, it


Figure 10. Scanning electron micrograph of gel grains exhibiting spherical humps on the surface. Product synthesized from gel with Si/Al = 0.99, at 225~ for 721 hours.

is not possible to hypothesize on a mechanism as suggested by Eggleton (1987) to explain the origin of hol- low packed spheres in non-crystalline Fe-Si-Al oxy- hydroxide gels, because the spheres synthesized during the hydrothermal experiments do not contain void spaces.

It is possible, however, that the mechanism could be similar to this (see Figure 11). During the early stage of runs a local and partial arrangement of rows of O-Si- O-AI-OH chains occur and kaolinite domains are formed. The formation of such domains might be favoured by the contraction of the gel, as it is documented by the change of leafy grains into compact grains (see particularly Figures 8 and 10). It is unlikely that the growing spherical particles were non-crystalline, or that the formation of a kaolinite structure occurred later, because in such a case a large variation in the chemical composition of the spherical particles should be expected. On the other hand, the formation of spherical structures in non-crystalline materials, such as volcanic glasses, is not uncommon and their presence has been documented by various authors, whatever the origin of the glass (Eggleton and Keller 1982, Tazaki et al 1989, 1992).

The reason why the kaolinite takes on a spherical shape is still unclear. It was suggested (Tomura et al 1983, 1985a) that the spherical morphology was the result o fa spherulitic aggregation of crystals. TEM observations (Figure 12) showed that the spherical particles have an undulated surface and light areas near the surface can be observed which might indicate the existence of round, less dense zones within the particle. It suggests that the mechanism might consist in the growth of domains with a kaolinite-like structure. The arrangement of groups of such domains might tend to be spherical, because this is the geometry correspond- ing to the min imum surface and the maximum volume. Unfortunately, we do not have data to confirm or reject

\l ll t= 1 ~ ~

t= 2 - - ~ . C ~ "~'-'-...Si:A1 ~- 1 " S i : A 1 ~ 1

t=3 ~ ~ .

Figure 11. Schematic drawing showing the hypothesized mechanism of formation of spherical kaolinite (see text).

such a speculation and further investigations should be carried out to deal with this topic.

As seen previously, the amount of spheres formed during the experiments was a function of the chemical composition of the gel; in particular many more particles were observed when the starting material had a Si/A1 ratio near 1. This observation apparently con- trasts with the result of Rodrique et al (1972) who affirmed that " . . . genesis of kaolin minerals is the easiest when the starting gels are characterized by a high silica content," but it should be taken into account that this affirmation was related to kaolinite with a well defined shape and not to spherical kaolinite. In any case, the reasons why the spherical particles a re spe- cially favoured in gels with Si/A1 ~ 1 are unclear. Re- cently Satokawa et al (1994) found that the structure of the gel influenced the morphology of kaolinite. Since gels with distinct Si/A1 ratios have different structures (Cloos et a l 1969), a role should be played by the chemical composition of the gel. It must be emphasized,

Figure 12. Transmission electron micrograph of some spheres showing their undulated surface (see text). Product synthesized from gel with Si/A1 = 0.99, at 225~ for 721 hours.


e l S . . . . . . . , , . - .

" % _ . . . . . . . . . . . . . - - �9

. . : . : ; : : / : .

:iii~i~:iii precipitation of lath~platy kaolinite . . . . . . . .

~ ~ k ~ release of spherical kaolinite :~

e r e d,

Gel is dissolved, became more compact as temperature and/or time increases. Hemispherical humps appear on the surface.

Platy and/or lath kaolinite precipitate from solution, dependant on the degree of supersaturation. Spherical kaolinite is released from gel; its quantity is controlled by the gel composition.

higher supersaturation

._g

: ) )

lower supersaturation

Different quantity of dissolved spherical kaolinite determine Si-AI concentration in the solution. Growth of lath kaolinite is favoured from a relatively higher concentration whereas platy kaolinite precipitates when concentration is lower.

Figure 13. Scheme of formation of spherical, lath and platy kaolinite from Si-A1 gel under hydrothermal conditions.

however, that the difference in behavior o f gel as a function of its chemical composi t ion may explain the results of the kinetic study by Huertas et al (1993b), who calculated that the activation energy of the first stage o f the process is not constant, but varies from 86.2 kJmol -~ for Si/A1 = 0.99 up to 117.5 kJmol - t for Si/A1 = 1.84.

In a previous study on the growth of kaolinite starting from gels with Si/AI = 1 (Tomura et al 1985b), the influence o f the solid/water ratio (S/W) on the precip- i tat ion o f spherical and platy kaolinite was suggested. Although in the present study S/W ratio was constant, in the light o f the data now acquired, speculative sug- gestions could be possible. During the earlier stage o f the runs, Si and AI are released from the gel to the solution and platy kaolinite may precipitate; at the same time, spherical kaolinite grows from the gel. I f S /W is high, the dissolution rate o f the gels is high (Lasaga 1981), supersaturation of the solution is reached in a short t ime and only a few platy crystals can precipitate because o f the unfavourable chemical condi-

tions in the medium. In fact, thermodynamic consid- erations (Van Oosterwyck-Gastuche and La Iglesia 1978) and laboratory data (Espiau and Pedro 1984) indicated that precipitat ion of the hexagonal platy kaolinite is favoured from solution having a low concen- trat ion of Si and AI. At the same time, a great number of spherical particles occur in the products o f the runs because their quanti ty does not depend on the chemical composi t ion o f the solution. On the contrary, i f S /W is low, the degree o f supersaturation unfavourable for precipitat ion of platy kaolinite, is reached over a larger t ime period and more platy crystals may precipitate. The proposed mechanism is also consistent with the suggestion by Tomura et al (1985b) that the presence of bohemite in the product of the run is at a very low S/W ratio. In fact, when S/W ratio is very low the saturation o f the solution is low enough to allow the precipitat ion of kaolinite, whereas bohemite crystal- lizes instead of kaolinite (Tsuzuki 1976, Walter and Hegelson 1977).

According to Tomura et al (1985b), the evolution of


/ . - . pLaitS j ~

�9 / /~ spheres /'x

/ , " x x / / "%.

decreases ~ 1 ~ . inca~eases

S i /A I a t o m i c ra t i o

Figure 14. Scheme of the distribution of morphological types as a function of the gel composition: at Si/A1 ratio near 1 more spherical particles and lath crystals are formed; at Si/ A1 far from 1 the quantity of the platy crystals is highest.

the sphere population with time and temperature is an indicator of the instability of the spherical kaolinite. It probably dissolves as runs progress because there is no evidence of the transformation into laths and/or plates through a solid state process.

The formation and the subsequent dissolution of the spherical particles is followed by crystallization of lath and/or platy crystals from the solution. This affirmation is consistent with Huertas et a! (1993b), who suggested that the formation of kaolinite from aluminosilicate gels is a two stage process. The first stage might correspond to the formation of spherical domains whereas the second two crystals having platy or lath morphology. Microscopic observations showed that the habit is not casual but is related to the chemical composition of the starting material: lath kaolinite is more frequent in products from gels with kaolinitic composition (Si/AI ~ 1), while platy kaolinite is prefer- entially formed in products from gels having a composition far from kaolinite. Since crystal morphology is controlled by chemistry of the solution (see Suna- gawa 1987, and references therein), it can be deduced that the solution in contact with the gel with kaolinitic composition have a higher chemical potential difference than the solution in contact with both Si-rich and Al-rich gels. Gel composition controls the chemistry of the solution but it is likely that also the quantity of dissolved spherical particles, which are more abundant in the earlier products from gel with Si/A1 ~ 1, plays a role in controlling the chemistry of the solution.

CONCLUSION

Mechanisms governing the transformation of aluminosilicate gels into kaolinite are summarized in Fig- ures 13 and 14 and the main features may be summarized as follows:

- -The overall reaction is the result of two different stages. During the first, the arrangement of the gel takes place and kaolinite-like materials are formed. The sec-

ond stage corresponds to the precipitation of kaolinite crystals from the solution.

--According to other authors, during the first stage of the transformation of gel under hydrothermal conditions, spherical kaolinite is formed. It is likely that spherical domains are formed and grow within the gel and later they are detached by dissolution of the gel.

- -The amount of the spherulitic particles occurring in the product of the runs depends on the composition of the starting gel: the highest amount of spheres occurs in the products from gel with Si/AI near 1.

- -The spherical particles are unstable and dissolve at a rate dependent on temperature and time. Their dissolution influences the chemistry of the solution which, in turn, controls the morphology of the mineral: more elongated, curved and irregular shaped crystals precipitate from solutions with a high chemical potential difference. As the highest amount of spheres are formed from gels with Si/A1 ~ l, in these runs lath crystals are more frequent than hexagonal crystals.

ACKNOWLEDGMENTS

This study has been supported by DGCYT (Project PB 88-093) and CNR. The CNR is also acknowledged for financing the SEM laboratory at the Dipartimento Geomineralogico (University of Bari, Italy), whose fa- cilities were used in the present work. TEM observations were done at Centro de Instrumentaci6n Cien- tifica of the University of Granada (Spain). The authors are grateful to R. E. Ferrel Jr. and the two referees, J. Elzea and K. Tomita, for the revision of the manu- script. R. Gautier is acknowledged for the revision of the English.

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(Received 5 July 1994; accepted 7 November 1994; MS. 2434)

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MORPHOLOGY OF KAOLINITE CRYSTALS SYNTHESIZED UNDER ... 43/43-3-353.pdf · more elongated, curved and irregular outlines when gels with Si/AI ~ 1 are hydrotherrnally treated. Key Words--Crystal