Growth of high temperature β-quartz from supercritical aqueous fluids

......... CRYSTAL

GROWTH

ELSEVIER

Journal of Crystal Growth 162 (1996) 142-146

Growth of high temperature fl-quartz from supercritical aqueous fluids V.S. Balitsky *, T.M. Bublikova, L.V. Balitskaya, A.G. Kalinichev Institute of Experimental Mineralogy, Russian Academy of Sciences 142432 Chernogolovka, Moscow Region, Russian Federation

Received i0 October 1994

Abstract

Methods have been developed for growing of/3-quartz crystals using gas and hydrothermal high-pressure vessels (10-12 ml internal volume), and autoclaves (20, 75, and 100 ml internal volume). The crystals were grown on bar-like a-quartz seeds at temperatures from 580 to 900°C and pressures from 0.5 to 5 kbar under isothermal and thermal gradient conditions. Pure water and aqueous solutions of NaOH, K2CO3, NH4F, A1F3, HF, Li3PO4, etc., were used as solvents. The nutrient was similar quartz bars or amorphous silica. Impurity elements (Fe, A1, P, Ti, Ge, etc.) were added to the nutrient in the form of oxides. It is shown that only the faces {10~0} and {1011}, as well as higher-indexed pyramidal {h0h/} faces were stable under the experimental conditions used. The growth rates of the {10~0} and {I0~ 1} faces were about the same ( ~ 0.02 mm/day), giving rise to isometric dipyramidal or prismatic habits. At temperatures above 600°C noticeable growth was observed under the temperature gradient conditions used even from fluids of rather low (0.05 to 0.15 g / c m 3) density. The intensity and direction of silica transfer substantially depended on temperature, temperature gradient, density and the alkalinity of the solutions, as well as on the fluoride ion concentration in acidic solutions. Impurity elements, except for Ge, were low ( ~ 0.001%). The results obtained in these experiments can provide an explanation for the growth peculiarities of /3-quartz crystals in miarole pegmatites and gas cavities of volcanic rocks.

1. I n t r o d u c t i o n

High-temperature /3-quartz is known to be stable at atmospheric pressures over the temperature range 573-870°C [1]. At a temperature below 573°C it rapidly transforms into low-temperature a-quartz. Above 870°C it very slowly transforms into tridymite. As the pressure increases, the temperature of the a - / 3 transformation also increases. For example, at pressures of 1, 5, and 9 kbars, the transformation temperatures are about 600, 700, and 800°C, respectively. Thus, there exists a wide region of /3-quartz

" Corresponding author.

stability relevant to the processes of mineral formation at medium and low depths of the Earth crust (some granitoids, pegmatites, high-temperature pneumatolith-hydrothermal veins), as well as on the surface (volcanic rocks, fumaroles). Practically the only criterion for quartz identification as the high-temperature fl-modification in granitoids and pegmatites is the estimated T - P conditions of their crystallization. At the same time, quartz phenocrysts in acidic volcanic rocks (rhyolites, liparites and dacites), along with the well-developed quartz crystals in the primary gas cavities of these rocks, can be safely identified as a-quartz paramorphs on /3-quartz [1]. The initial growth zones of pneumatolith-hydrother-

0022-0248/96/$15.00 © t996 Elsevier Science B.V. All rights reserved SSD1 0022-0248(95)00939-6

V.S. Balitsky et al./Journal of Crystal Growth 162 (1996) 142-146

mal "cellular" quartz crystals from the cavities of miarole pegmatites are also identified as a-quartz paramorphs on /3-quartz [2]. From the mineralogical and crystallographical point of view these paramorphs have been studied in sufficient detail [1]. However, the effect of the physical-chemical and growth factors on the morphology and internal structure of /3-quartz crystals, their real texture, the kinetics of face growth, and distribution of impurity components is still not clear. Such data are important for understanding the genesis of the above-mentioned rocks and could be directly obtained through growth experiments under controlled conditions, as it has been earlier shown for low-temperature quartz [3]. The possibility of growing /3-quartz crystals from supercritical aqueous fluids is supported by the data of quartz solubility in pure water and aqueous electrolyte solutions at the T - P parameters of /3-quartz stability [4,5]. However, such crystals have not yet been grown on seeds, although the synthesis of fine-crystalline/3-quartz is readily realized by recrystallization of amorphous silica [1] and crystobalite [6]. The present paper reports on the results of the first successful experiments on the growth of /3quartz on seeds from supercritical aqueous fluids over the temperature range between 580 and 900°C and at pressures from 0.5 to 5 kbar.

2. Experimental procedure The crystals were grown using gas and hydrothermal high-pressure vessels and heat-resistant autoclaves designed at the Institute of Experimental Mineralogy, Russian Academy of Sciences. When gas and hydrothermal high-pressure vessels were used, the crystals were grown using inner platinum capsules (diameter: 6 - 8 mm; length: 80100 mm) under isothermal and thermal gradient conditions. Thin bars cut out of a synthetic c~-quartz single crystal were used as seeds. They had rectangular cross sections (3 × 3 mm) and length up to 70 mm in the [1120] crystallographic direction (so called x-bars). Their longer surfaces were parallel to the pinacoidal face {0001} and the face of the hexagonal prism {1010}, respectively. This choice of seed geometry allowed us to obtain the most comprehensive

143

information on the growth and morphological characteristics of the crystals grown. In the thermalgradient experiments, a part of the quartz bar located within the relatively higher-solubility zone of the capsule played effectively the role of nutrient for the other part of the same bar located within the growth zone of the capsule. In the isothermal experiments, freshly prepared and annealed (at 600°C) amorphous silica was used as a nutrient. In both types of experiments pure water was used as a solvent. The capsules were first filled with the nutrient and the seed quartz bar (or with the bar alone), then water was added, with a filling factor sufficient for the platinum capsules to remain sealed under the conditions of experiments. External pressure was created by water or argon depending on the highpressure apparatus used. Hydrothermal pressure vessels were used for experiments at 3 kbar and at temperatures from 580 to 680°C, while gas highpressure vessels - for experiments at 5 kbar and from 780 to 900°C. When autoclaves (20, 75, and 100 ml in volume) were used, the crystals of /3-quartz were grown by the temperature gradient method. Two similar single crystalline a-quartz x-bars were placed, respectively, over a perforated diaphragm within the upper (relatively lower-temperature) zone of the autoclave, and under the diaphragm at the bottom of the autoclave. This allowed us to monitor simultaneously the growth and the dissolution of the single crystals, as well the direction of silica transfer depending on the chosen temperature, pressure, and fluid composition. The crystals were grown at temperatures 600, 650, 680, and 720°C and pressures from 0.5 to 1.8 kbar (estimated using the P - V - T diagram for pure water). The temperature difference between the bottom and the top zones of the autoclave was kept from 20 to 60°C. Starting solutions were either pure water or low concentration (0.01-1 wt%) aqueous solutions of NaOH, K2CO3, NH4F, HF, and AIF3, which allowed us to vary the pH value between 10 and 1. In some experiments, oxides of metals most abundant as isomorphic substitutes in the quartz structure (AI, Ti, Fe, P, Ge) were added to the nutrient. The crystals grown were studied under stereoscopic binocular and polarization microscopes. The morphology of the crystals, the relief and other details of their faces were investigated. The internal

144

V.S. Balitsky et al. / Journal of Crystal Growth 162 (1996) 142-146

structure of the crystals and growth rates of the faces were studied on thin ( < 1.5 mm) polished wafers cut off perpendicular to the faces of the hexagonal prism. The content of the impurity components was determined using a "Camebax MBX" microprobe and an energy-dispersive spectrometer "Link 8 6 0 / 5 0 0 " . The presence of radiation coloration centers was detected by means of ",/-irradiation of the crystals (source 6°Co, dose 5-10 mrad).

3. Results and discussion

A total of more than 70 /3-quartz crystals were grown on seeds (Fig. 1). They all underwent transformation to the low-temperature a-modification upon cooling of the pressure vessels and autoclaves below the temperature of c~-/3 equilibrium. Nevertheless, there are several evidences that the crystals were initially grown as /3-quartz. First of all, the crystal growth occurred directly within the P - T field of /3-quartz thermodynamic stability. Secondly, the relief of faces of the positive

I

J

Fig. 1. A typical crystal of a-quartz paramorph on r-quartz (left) grown from a bar-like x-seed in supercritical aqueous fluids. On the right there is a partially dissolved quartz bar from the nutrient autoclave zone. Scale bar indicates 10 mm.

{10~1} and negative {01~i} rhombohedra of the resulting a-quartz paramorphs on /3-quartz was absolutely identical and their growth rates were equal. (This can indicate that under the conditions of experiments these faces of a-quartz crystals had the same index, namely, the one of the hexagonal pyramid {I011}.) Finally, all crystals were prone to cracking similar to the so-called "cellular" cracking characteristic of the natural r-quartz transformation into the low-temperature a-modification [2]. Other types of cracks occurred in the seeds during the initial heating of the pressure vessels, before the conditions for/3-quartz crystal growth were achieved inside the autoclave. They were easily diagnosed, because individual cut blocks form along them during further growth. The nutrient quartz bars were also hardly affected by cracking (see Fig. 1). Similar to a-quartz, the most intensive growth of /3-quartz crystals was observed on the pinacoidal surface {0001}. In neutral and alkaline solutions this surface is unstable and gets covered with the smallest tightly placed pyramids oriented as the faces of the hexagonal pyramid {1011}. Geometric selection during the growth process results in their coarsening. However, in the crystals grown from acidic fluoride solutions, the pinacoidal surface has an unusual cellular structure of the negative "cobbles" type. The rate of the pinacoidal surface regeneration in different experiments varied from 0.1 to 0.5 ram/day and increased with increasing T - P parameters, temperature gradient, and alkalinity of solutions, and for acidic solutions - with an increase in fluorine concentration. The regeneration of the pinacoidal surface was accompanied by the appearance of the hexagonal prism {1010} and hexagonal pyramid {1011} faces. They grow as smooth faces and have about the same growth rates, which is a factor of 5-10 lower than that of the pinacoidal surface. In the process of growth, between the faces {1011} and {1010} there often appear higher-indexed pyramidal faces {h0h/}. Small (0.2-2.0 mm) crystals, which were formed spontaneously and simultaneously with the crystal growth on seeds, always had an isometric habit with predominantly hexagonal pyramidal faces {10~ 1} and much less developed prismatic faces {1010}. Less often the spontaneous formation of crystals with elongated (parallel to the optical axis) habit was observed. The elongation was due to the appearance

V.S. Balitsky et al. / Journal of Crystal Growth 162 (1996) 142-146

145

Fig. 2. The relief of the {10~0} crystal faces of a-quartz paramorph on /3-quartz grown from supercritical aqueous fluids. Scale bar indicates 10 mm.

of the higher-indexed pyramidal faces {h07~l}, as it was recently noted by Hosaka et al. [6]. The {10~1} faces had a vicinai relief. Vicinal forms look like small asymmetrical slightly sloping hillocks of spiral structure. The {10~0} faces featured shadings parallel to the edges between {1010} and {1071} faces (Fig. 2), typical to the natural quartz crystals. As the duration of experiments increased, all stages of the crystal formation on the seeds could be observed: from an incomplete regeneration of the pinacoidal surface to its complete pinching-out, followed by the overgrowing by the growth sectors (10~0) and (1011) (Fig. 3). The regenerative character of the pinacoidal surface growth led to the formation of the extremely non-uniform (0001)

i

I

~!ii! ~i!!!iiiiiiiii~;i!i!iiiii!ii¸ iiiiiii~ ~. . . . . .

growth sector composed of parasitic growth sectors (10~1) (Fig. 4). The (1010) and (10~1) growth sectors are represented by more uniform layers, but, as a rule, they are subject to twinning. The (0001) sectors are uniform only when the growth takes place from acidic fluoride solutions (Fig. 5). The metal oxides added to the system did not have any noticeable effect on the crystal growth characteristics. Except for germanium, which was

'y

k (a)

~!!~ji?i~iiiii!iii~iiiiiii~

Fig. 4. The internal structure of the (0001), (1010) and (10~1) growth sectors of an a-quartz paramorph on /3-quartz grown from a weakly alkaline aqueous fluid (zy-section). Scale bar indicates 1 mm.

(b)

(c)

Fig. 3. The change of the overgrown layer in the process of crystal growth on the bar-like x-seeds (zy-section): (a) 10 days; (b) 17 days; (c) 24 days. In case (c), the growth sectors (10~0) and (10"il) are seen to appear. Scale bar indicates 1 mm.

Fig. 5. The internal structure of the (0001) growth sectors of an a-quartz paramorph on /3-quartz grown from an acidic (pH = 1) fluorine-bearing fluid (zy-section). Scale bar indicates 1 ram.

146

V.S. Balitsky et al. /Journal of Crystal Growth 162 (1996) 142-146

detected in the grown crystals in concentrations up to 8 wt% GeO 2, the content of all other impurities (Fe, AI, Ti, P) was of the order of 10 -3 wt%. 3,-Irradiation of the crystals revealed the presence of Al-alkaline centers of smoky color only. The intensity of the coloration increases with increasing alkalinity of the solvent. Somewhat unexpected, under direct temperature gradient conditions for temperatures between 600 and 900°C, there was observed a noticeable silica transfer and quartz crystal growth from pure water and alkaline solutions of very low density (filling factors for the autoclaves were 10%-15%). It is known [7] that at temperatures of 450-500°C, the direction of silica transfer in low-density aqueous fluids (0.2-0,3 g / c m 3) under thermal-gradient conditions is reversed, i.e. from the top (relatively lower-temperature) to the bottom (high-temperature) zone of the autoclave. In our experiments, at temperatures above 600°C and with pure water or alkaline solutions as solvents, such reversed character of silica transfer and crystal growth were observed only with filling factors below 10%. At the same time, it should be emphasized that in acidic fluoride solutions within the wide density range studied (filling factors from 5% to 30%) the direction of silica transfer was reversed in the above sense. This phenomenon can be explained by the presence of relatively higher HF concentrations (and, consequently, higher silica content) in the upper zone of the autoclave under the direct temperature gradient conditions. As was expected, the intensity of the silica transfer, hence the /3-quartz crystal growth rates, increased with increasing temperature, pressure, temperature gradient, and initial pH of the solvent.

low-density ones. In pure water and alkaline fluids, at temperatures above 600°C the direction of silica transfer leading to the crystal growth coincided with the direction of the temperature gradient, down to densities of about 0.1-0.15 g / c m 3. At lower densities the direction of the silica transfer was reversed. In acidic fluoride solutions this reversed character of silica transfer takes place at higher densities of the fluids (up to 0.3 g/cm3). Over the wide range of temperatures and pressures studied, only prismatic {1010} and pyramidal {1011} and {h0hl} faces remained stable and developed. The growth rates of {1010} and {1011} faces were about the same ( ~ 0.02 ram/day), providing for the hexagonal stout prismatic or pyramidal habit similar to that of natural /3-quartz crystals. Despite the high T - P parameters of crystallization, the structure of the /3-quartz grown in this study remains closed to the common impurity elements (except germanium) as does the a-quartz structure does.

4. Conclusions

[3] V.S. Balitsky, ExperimentalStudy of the Processes of Crystal Formation (Nedra, Moscow, 1978) (in Russian). [4] G.C. Kennedy, C.J. Wasserberg, H.C. Heard and R.C. Newton, Am. J. Sci. 260 (1962) 501. [5] G.M. Anderson and C.W. Burnham, Am. J. Sci. 263 (1967) 12. [6] M. Hosaka,T. Miyataand I. Sunagawa,J. Crystal Growth 152 (1995) 300. [7] M. Hosakaand S. Taki, J. Crystal Growth 51 (1981) 640.

In the present study fl-quartz single crystals were for the first time grown on seeds from supercritical aqueous fluids. It was demonstrated that, unlike low-temperature a-quartz, they can form under the conditions of a direct temperature gradient not only from relatively dense fluids, but also from very

Acknowledgements The authors are grateful to Drs. H. Iwasaki and F. Iwasaki for helpful discussions. The valuable comments of the anonymous reviewer are also greatly appreciated. This work has been supported by the Russian Basic Research Foundation (Grant No. 9405-17453a).

References [I] C. Frondel, The System of Mineralogy, Vol. III, Silica Minerals, 7th ed. (Wiley, New York, 1962). [2] Yu.A. Dolgov, in: Conditions of formation of piezo-optical minerals in pegmatites (Nedra, Moscow, 1969) pp. 3-21 (in

Russian).