Some mineralogical equilibria in the system K 2 O-Al 2 O 3 -SiO 2 -H 2 O

Abstract

An experimental investigation was made at elevated temperatures and pressures of the hydrolysis of K-feldspar to mica+silica and of mica to kaolinite. Values of the equilibrium quotient, m _KCl /m _HCl , were determined at temperatures ranging from 200 degrees to 550 degrees C. The solution pressure for most of the runs was 15000 p.s.i. although determinations were made 25 at both higher and lower pressures. The reactions are exothermic, and the equilibria show a marked shift to higher acidities with increasing temperature. The equilibrium curves for the two reactions are essentially parallel. Above about 350 degrees C. the assemblage mica-kaolinite changes to mica-pyrophyllite-boehmite, and at still higher temperatures mica-pyrophyllite-andalusite is formed. At 400 degrees C. and 15000 p.s.i. the equilibrium quotient is 10 (super 2.7) for the assemblage K-feldspar-mica-silica, and 10 (super 1.3) for the assemblage mica-pyrophyllite-boehmite. The experimental equilibrium quotients probably do not differ from the thermodynamic equilibrium constants of the reactions by more than a factor of five up to temperatures of 350 degrees C. or higher. The experimentation shows that the most important controls on the fields of stability of these several minerals are the K (super +) /H+) activity ratio and the temperature at a given temperature and with increasing K (super +) /H+) ratio, the fields of kaolinite, mica and K-feldspar are successively traversed. Similarly, at constant K+/H+) ratio with increasing temperature this same sequence is observed. The free energy drive of hydrolysis decreases with increasing temperature. Thus the alteration potential of hydrothermal solution would increase with decreasing temperature provided the rate of migration were sufficiently rapid in comparison with the rate of reaction with the wall rock. Experiments at solution pressures of 5000 and 35000 p.s.i. show that the effect of solution pressure on these equilibria is relatively small. An increase in pressure increases the extent of hydrolysis, whereas a decrease in pressure shifts the equilibria in the opposite direction. The experimental results have particular application to hydrothermal alteration and the genetic interpretation of various alteration mineral patterns.