The crystallization of feldspar in volcanic acid liquids

Abstract

The paths of feldspar crystallization in volcanic acid liquids can be illustrated by the relationship of feldspar phenocrysts to groundmass feldspar in porphyritic pitchstones and rhyolites. In this way, a concept of the undetermined system CaAl₂Si₂O₈-NaAlSi₃O₈-KAlSi₃O₈-SiO₂ can be obtained. In this account the various sub-solidus transformations of alkali feldspar and plagioclase are neglected, the emphasis being rather on feldspar-liquid equilibria. It is shown that the equilibrium crystallization of acid liquids similar in composition to the Icelandic rhyolites is characterized by a single feldspar, namely a representative of the plagioclase-anorthoclase solid-solution series. In the more potassic acid liquids of the British Tertiary and the San Juan region, Colorado, phenocrysts of alkali feldspar (sanidine) and plagioclase are commonly found, plagioclase being subsequently resorbed under certain conditions during equilibrium crystallization.