pargasite

Pargasite is a mineral in the amphibole group, classified as a sodium-calcium amphibole. It is part of the hornblende family and is characterized by a double-chain inosilicate structure with hydroxyl groups. Its composition is variable, but pargasite crystals typically contain sodium, calcium, magnesium, iron and aluminum, with silicon-oxygen tetrahedra forming the characteristic amphibole framework. The solid-solution series includes related endmembers such as ferropargasite and edenite, reflecting variations in Na, Ca, Mg and Fe content.

Pargasite commonly forms in metamorphic rocks that have experienced high-grade metamorphism, including schists, granulites and granulite-facies rocks, as well as in regional metamorphism of pelitic rocks. It may occur together with hornblende (magnesio-hornblende), biotite, garnet and plagioclase, and can also appear in hydrothermal veins.

The mineral is typically green to brown-green, with a vitreous to pearly luster, and crystallizes in elongated prismatic crystals. It has a Mohs hardness around 5–6 and a specific gravity between 3.2 and 3.5. Like other amphiboles, pargasite shows two directions of cleavage near 56 and 124 degrees and exhibits strong pleochroism.

The name pargasite derives from the town of Pargas (Parainen) in Finland, where the mineral was first described. It is used in geology as a petrographic indicator of metamorphic conditions, and its presence helps distinguish different metamorphic facies and pressure-temperature histories in rocks.