SiO2 H2O System Research Articles

A hydrothermal experiment for retrograde change of gehlenite.

Phase relations in the system gehlenite (Ak0Ge100 or Ak20Ge80)–SiO2–H2O were investigated at 1 kbar water pressure and at temperatures from 350° to 750°C. Crystalline phases formed in the gehlenite end-member (Ak0Ge100)–SiO2–H2O system are bicchulite, garnet, anorthite and corundum (or boehmite), and those in the gehlenite solid solution (Ak20Ge80)–SiO2–H2O system are vesuvianite, xanthophyllite and diopside in addition to the phases formed in the end-member system. The results of the solid solution system are more consistent with the natural alteration reactions than the results of the end-member system.The coexisting crystalline phases observed in the solid solution system change successively with the fall of temperature as follows: (1) gehlenite + garnet, (2) gehlenite + vesuvianite ± garnet, (3) bicchulite + xanthophyllite + vesuvianite and (4) bicchulite + vesuvianite ± garnet. The assemblage (1) is stable at temperatures above 720°C and that of (2) is stable at temperatures between 610°C and 720°C. The stability boundary between the assemblages (3) and (4) is inclined: 340°C for no silica-addition, 450°C for the added silica/gehlenite = 0.03 and 500°C for the added silica/gehlenite = 0.06. The large amounts of silica-additions result in the assemblage of garnet + anorthite ± diop-side at temperatures between 350°C and 750°C.Cell dimensions of hydrogrossular, i.e., degrees of hydration, vary depending not only on the formation temperatures but also on silica deficiencies of the starting materials. Cell dimensions of vesuvianite vary in the same manner as grossular. A linear relationship between the cell dimensions of garnet and those of vesuvianite was observed in the present experiment.Examples consistent with the present experimental results are found in natural mineral assemblages from many localities. The alteration conditions of gehlenites at Crestmore and Kushiro, correspond to the mineral assemblage (2), whereas those at Akagane mine corresponds to the assemblage (3). The alteration conditions which were commonly observed at Fuka correspond to the assemblage (4). The experimental results may be applied to processes and conditions of natural alteration reactions of gehlenite at other localities as well. The same correlation between the cell dimensions of garnet and those of vesuvianite as in the synthetic experiments is seen in natural specimens from Fuka and Kushiro.

Measurement of the Near-Infrared Spectra of H2O-SiOH-Bearing Siliceous Materials at High Pressures

Techniques for measuring OH-combination bands of water and other OH-group bearing solid or liquid materials in the near-infrared (6000 to 4000 cm−1) at high pressures by means of a modified diamond anvil cell, adapted to a single beam micro-spectrometer, have been developed and applied to opals, i.e., minerals in the system SiO2-H2O containing relatively high amounts of water and SiOH. Liquid embedding, using the gasket technique and poly-fluoro-carbon oils as embedding liquids, which provide a hydrostatic pressure environment at least up to 50 kbar, is applied for solid materials. The high pressure near-infrared spectra of gel-like, spherulitic opal-AG show a strong relative decrease of the high energy components of the complex (v1 + v2)H2O-band system at around 5200 cm−1 in favor of the low energy components of this band system. This is tentatively interpreted as due to an increase of fully hydrogen bonded water at the expense of water in which only one OH-group is H-bonded. The corresponding effect in opal-AN with a glass-like continuous SiO4/2-tetrahedral network is much smaller.

The synthesis, characterization, and catalytic activity of omega and ZSM-4 zeolites

The zeolites omega and ZSM-4 were both synthesized in the [(CH3)4N]2ONa2OAl2O3SiO2H2O system at a temperature of 95 °C. Although the starting reagents and synthesis procedures are different, the omega and ZSM-4 zeolites, on the basis of X-ray diffraction, have similar aluminosilicate structures. In the omega synthesis, a faujasite → omega reconstructive transition was discovered. No such transition was found in the ZSM-4 synthesis. Adsorption measurements show that omega zeolite rapidly adsorbs cyclooctatetraene but not (C4F9)3N. Acid catalysts derived from omega exhibit high α values for cracking of n-hexane. Hydrogen and lanthanum forms of omega and cocrystallized omega and faujasite show an increase in catalytic cracking activity with an increase in faujasite content. Comparative studies between omega and ZSM-4 catalysts show a slightly higher catalytic cracking activity for omega zeolite.

The transformation of amorphous silica to crystalline silica under hydrothermal conditions

Hydrothermal syntheses were made mainly in the binary system SiO2-H2O in a temperature range between 300 ° C and 500 ° C and pressures from 0.2 kbar up to 4.0 kbar with various starting materials. In this way the transformation behavior of different amorphous silicas via cristobalite and keatite to quartz were observed. This behavior depends mainly on the parameters: pressure, temperature, run duration and state of the starting material. Four reaction paths have been observed: in most experiments the complete reaction sequence “amorphous silica→cristobalite→keatite→quartz” took place. Less often the reactions: “amorphous silica→cristobalite→quartz” and: “amorphous silica→keatite→quarts” were observed. Very few samples were found with a direct transition of amorphous silica into quartz at high pressures. A kinetic model is given in form of a pressure-temperature-time diagram of the system SiO2-H2O under hydrothermal conditions.