Hornblende

Hornblende is a group name for dark-colored amphibole minerals, common in igneous and metamorphic rocks. The name derives from German Hornblende, meaning horn blade, in reference to its elongated crystals.

Composition and structure: Hornblende belongs to the inosilicate (amphibole) family and is a double-chain silicate. It is a complex solid solution primarily containing calcium, magnesium, iron, and aluminium, with silicon dioxide and hydroxyl groups forming (Si8O22)(OH)2 units. The precise composition varies, giving rise to a series that includes ferrohornblende and magnesiohornblende; sodium- or potassium-rich varieties exist in some rocks.

Occurrence: It occurs in plutonic and volcanic rocks such as diorite, andesite, granite, and schists, as well as in metamorphic rocks formed under intermediate to high-grade conditions. It is especially common in amphibolite facies metamorphism and often forms prismatic crystals with good cleavage.

Physical properties: Hornblende crystals are typically dark green to brown-black and opaque to translucent. They have a vitreous to sub-metallic luster, a Mohs hardness around 5–6, and two cleavage directions at approximately 56 and 124 degrees, giving a characteristic parallel fracture.

Significance and uses: Hornblende is a useful mineral in rock classification and geochronology. It can host potassium, allowing radiometric dating (K-Ar and Ar-Ar methods) to determine the ages of rocks containing it. Its presence helps define metamorphic grade and informs about the tectonic history of regions.