The hydrothermal alteration of feldspars at 250 degrees C. to 400 degrees C

Abstract

About 40 experiments with feldspars were made in K 2 S solutions and in basic, neutral, and acidic concentrated solutions of NaCl or KCl at 250 degrees C. to 400 degrees C. Alumina or silica was added in some experiments. It was found that in the presence of excess K ions potash and soda feldspars alter to kaliophilite in solutions more basic than about 0.10 M K 2 S and 0.08 N KOH. In less basic solutions and in solutions as acidic as 0.001 N potash feldspar alters to soda feldspar in the presence of an excess of Na ions. The reaction can be reversed in the presence of an excess of K ions. For a short range of concentrations more acidic than 0.001 N soda feldspar is stable for the time of the experiments, but in 0.020 N HCl soda feldspar alters to pyrophyllite. In the presence of alumina and excess K ions soda feldspar alters to muscovite in solutions as basic as 0.010 N. In the presence of silica substitution of Na in labradorite was noticeable in 0.050 N NaOH and nearly complete in 0.150 N NaOH. In less basic and in acidic solutions no Na substitution was obtained. The writer has attempted to explain some of the reactions.