Mineralogy and Mineral Stabilities

Abstract

As noted in Section 4.2, andesites are usually porphyritic in texture, thereby providing a rich mineralogic record of their history, as well as sometimes providing Earth’s finest examples of anorthite and of coexisting pigeonite and hypersthene. However, most studies of andesite report only the identity or, at most, the proportion of phenocrysts present. Indeed, the isotopic composition of Sr is known for more andesites than is the composition of their conspicuous pyroxene phenocrysts! The data summarized below broadly constrain the physical conditions of phenocryst crystallization but are insufficient to make possible a systematic investigation of relationships between mineralogy and andesite type, eruption history, or the tectonic environment of eruption. In addition, Ewart (1976a, 1979, 1980) has summarized relationships between modal proportions of phenocrysts, bulk compositions, and crustal environments.