amphiboleplagioclase

Amphibole-plagioclase is a descriptive term used for rocks in which amphibole minerals and plagioclase feldspar are the dominant mineral phases. It is not the name of a single mineral, but a common rock assemblage found in both igneous and metamorphic rocks.

Amphiboles are a group of inosilicate minerals with double-chain structures, including hornblende, tremolite, actinolite, and related species. They typically contain calcium, sodium, iron, magnesium, and aluminum in varying proportions. Plagioclase is a feldspar solid solution between albite (sodium-rich) and anorthite (calcium-rich), forming a continuous range of compositions. In rocks labeled amphibole-plagioclase, these two mineral groups occur in substantial and texturally interrelated amounts.

In metamorphic rocks, the amphibole-plagioclase assemblage is characteristic of amphibolite facies and often coexists with quartz, biotite, and sometimes garnet or pyroxene, depending on pressure and temperature. The specific Fe-Mg and Ca-Na exchange between amphibole and plagioclase provides information about the metamorphic conditions. In igneous rocks, hornblende-bearing granitoids, diorites, and andesites commonly display a prominent amphibole-plagioclase suite, reflecting the crystallization history from silica-rich magma.

Significance: the amphibole-plagioclase assemblage is widely used in metamorphic petrology to infer P–T conditions and tectonic settings, and to correlate metamorphic histories with igneous activity. Textures and chemical compositions of these minerals help reconstruct crystallization sequences and deformation histories within the rock.