Kinetics of alteration of K-feldspar and its application to the alteration zoning

Abstract

K-feldspar was treated with acid solutions containing various concentrations of H⁺ and K⁺ at 210°∼270°C in silica glass tubes. Amounts of Si, Al and K in the solutions were determined after each experimental run. The alteration products comprised kaolinite in general, although sometimes a 12Å mineral appeared and only amorphous material was observed in strong acid solutions. The rate of reaction depends on the chemical composition of solution and the temperature. On the basis of the experimental results, the alteration process can be expressed by the following rate formula, α = 1 - exp(-ktⁿ) where α is the fractional amount of kaolinite in all the minerals present, t is time, and k and n are constants. On the assumption that Al is an inert component, that SiO₂ behaves as a mobile component capable of being dissolved, migrating in solution and being precipitated out of it, and that K is mobile and flows out of the system, the zonal arrangement of inner kaolinite zone and outer silicified zone, which can be observed in some kaolin deposits, is illustrated numerically in a model involving a kinetic process.