Rate of clay formation and mineral alteration in a 4000-year-old volcanic ash soil on Saint Vincent, B.W.I

Abstract

A pyroclastic deposit fan has weathered to a 6-ft. clayey topsoil at 1.5-2 ft./millennium. Glass decomp. at 15 g./sq. cm./millennium. In andesitic ash of glass, anorthite, labradorite, hypersthene, augite, and olivine, there is alteration to halloysite tetrahydrate which, particularly the lapilli of altered fine-vitrics, may redeposit as braunlehm. Fine vitrics are also changed to allophane and hydrous ferric oxide. Anorthite is etched in soil and frosted in unaltered ash. The soil holds etched hypersthene and some of its augite is etched. Labradorite, hornblende, and magnetite, but not olivine, are unaltered. The slow rate of anorthite, pyroxene, and olivine alteration contrasts with the glass decompn. rate. [on SciFinder(R)]