anthophyllite

Anthophyllite is a magnesium-iron inosilicate mineral in the amphibole group. Its chemical formula is commonly written as (Mg,Fe)7Si8O22(OH)2, indicating that magnesium and iron substitute for one another within the crystal structure. The mineral crystallizes in the orthorhombic system and typically occurs as elongated prismatic crystals or as fibrous, asbestos-like aggregates. It has two cleavages at about 56 and 124 degrees, a characteristic feature of amphiboles, and generally ranges in color from colorless to gray-green to brown. Hardness is about 5.5–6 on the Mohs scale, and it forms a vitreous to silky luster.

Anthophyllite forms under metamorphic conditions, especially in ultramafic rocks that have undergone significant metamorphism, such as serpentinites and talc-rich schists. It can occur as an accessory mineral in contact or regional metamorphic environments and is commonly found alongside minerals such as olivine, enstatite, diopside, talc, chlorite, and magnetite. In nature, anthophyllite is relatively rare compared with other amphiboles, and its presence can indicate specific pressure–temperature histories in the rock.

The fibrous variety of anthophyllite is known as anthophyllite asbestos. Like other asbestos minerals, this form is hazardous when its fibers are inhaled and is subject to strict health and safety regulations. Outside of asbestos contexts, anthophyllite is mainly of interest to mineral collectors and as a mineralogical indicator in metamorphic petrology.