Andesite porphyry is an intermediate volcanic rock that contains phenocrysts (larger crystals) embedded in a fine-grained groundmass. It is classified as a porphyry because of this characteristic two-phase texture. The mineral composition of andesite can be quite complex and variable. This complexity arises from the conditions under which the rock formed.
The phenocrysts in andesite porphyry typically consist of plagioclase feldspar, amphibole, and pyroxene. Plagioclase feldspars are tectosilicate minerals that make up the continuous solid solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). The specific plagioclase composition depends on the original magma composition. Amphiboles are inosilicate minerals containing iron, magnesium, calcium, sodium, and other cations. Common amphibole minerals found in andesite include hornblende and actinolite. Pyroxenes are also inosilicate minerals but with a different crystal structure. Augite and pigeonite are typical pyroxenes in andesite porphyry.
The groundmass of andesite porphyry consists of fine-grained crystalline material along with any volcanic glass that did not fully crystallize. It frequently contains the same minerals as the phenocrysts along with additional ones like biotite, quartz, alkali feldspar, titanomagnetite, and apatite. Biotite is a sheet silicate mica mineral rich in iron, magnesium, and aluminum. Quartz is silicon dioxide and is one of the most abundant minerals in Earth's continental crust. Alkali feldspar refers to potassium-rich tectosilicate minerals like orthoclase and sanidine. Titanomagnetite is an oxide mineral that contains iron, titanium, and oxygen. Apatite is a phosphate mineral composed of calcium, fluorine, chlorine, and hydroxyl ions.
The particular mineral assemblage of any given sample of andesite porphyry depends on the original composition of the magma from which it crystallized as well as the pressure and temperature conditions during solidification. More felsic (silica-rich) magmas produce andesite with increased amounts of quartz and alkaline feldspar in the groundmass. More mafic (iron and magnesium-rich) magmas result in more amphiboles and pyroxenes. Higher pressure during crystallization generally increases the calcium content of plagioclase feldspar phenocrysts. The complexity of andesite porphyry mineralogy makes it an interesting rock to study for geologists seeking to understand the geologic processes at convergent plate boundaries where andesitic magmatism predominates. Careful petrographic and chemical analysis of the component minerals can yield valuable information about the genesis of these common volcanic rocks.
