SILICA TRANSFER AND BETA-QUARTZ GROWTH FROM SUPERCRITICAL AQUEOUS FLUIDS

Methods used to grow beta-quartz crystals include, gas and hydrotherma l 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 lpha-quartz seeds at temperatures up to 900 degrees C and pressures fr om 20 to 500 MPa, under isothermal and thermal gradient conditions. Pu re water and aqueous solutions of NaOH, K2CO3, NaCl, NH4F, AlF3, HF, L i3PO4 were used as solvents. The nutrient was similar quartz bars or a morphous silica. Impurity elements (Fe, Al, P, Ti, Ge etc.) were added to the nutrient in the form of oxides. The details of silica transfer during the crystal growth process, under supercritical conditions, ha ve been analyzed. It was demonstrated for the first time that in pure water and nearly neutral or alkaline fluids, the direction of silica t ransfer always coincides with the direct temperature gradient. However , in acidic fluoride fluids and pressures below 80-100 MPa, the direct ion of silica transfer undergoes inversion and is opposite to the dire ct temperature gradient. This phenomenon is related to the nonmonotono us and relatively strong temperature and density dependence of the HF dissociation constant which affects quartz solubility. The intensity a nd direction of silica transfer is substantially dependent on temperat ure, temperature gradient, density and alkalinity of the solutions, as well as being dependent on the fluoride ion concentrations in acidic solutions. It was shown that only faces {1010} and {1011}, as well as higher indexed pyramidal {h0hl} faces are stable under experimental co nditions. The growth rates of faces {1010} and {1011} are about the sa me (similar to 0.02 mm day(-1)). Impurity elements, except for Ge, are scarce (similar to 0.001%) in the quartz structure. The results obtai ned provide an explanation for the growth peculiarities of beta-quartz crystals in natural environments. (C) 1998 Elsevier Science B.V.