Enhanced Al/Si Diffusion in KAlSi3O8at High Pressures: The Effect of Hydrogen

Abstract

The effect of pressure on the rate of Al/Si disorder in microcline has been determined at temperatures from 750°C to 900°C and at pressures up to 28 kbar using samples dried at 850°C in Pt capsules prior to sealing. The presence of any water would induce melting, and none was observed. The effect of pressure in the presence of hydrogen is profound in experiments done in piston-cylinder devices using NaCl pressure medium, as has also been observed with albite. The enhanced Al/Si diffusion is even more marked than in albite, although a somewhat higher threshold pressure is necessary to attain fairly rapid rates of Al/Si interchange; disorder is very slow in this temperature range at pressures below 9-10 kbar. At 1050°C and one atmosphere, an authigenic microcline from the Kona dolostone is hardly changed in six weeks. At 900°C and 28 kbar it is disordered in less than two hours. As in the case of albite, the presence of hydrogen is essential for the enhanced diffusion at high pressures; if deprived of hydro...