amphiboles

Amphiboles are a large group of inosilicate minerals characterized by a double-chain arrangement of SiO4 tetrahedra. In the crystal structure, tetrahedra share oxygen atoms to form two joined chains, which are then linked by various metal cations and hydroxyl groups. This double-chain framework distinguishes amphiboles from single-chain pyroxenes and accommodates a wide range of chemical compositions, typically including calcium, magnesium, iron, aluminum, and sodium.

Most amphiboles are hydrous silicates that contain hydroxyl (OH) groups. They occur in a variety of environments, but are especially common in metamorphic rocks formed under greenschist to granulite facies and in hydrothermally altered igneous rocks. Major rock associations include amphibolites, schists, and andesites and diorites with hornblende.

Physical properties of amphiboles include a tendency to form elongated prismatic crystals, colors that range from white to green, brown, or black, and two cleavage directions at approximately 56 and 124 degrees. They have a Mohs hardness of about 5–6 and may be monoclinic or orthorhombic, often showing pleochroism depending on composition.

Common members and significance: tremolite, actinolite, hornblende, and anthophyllite are well-known amphiboles. Some amphiboles occur as asbestos minerals, including crocidolite and amosite. Amphiboles are important in petrology as indicators of metamorphic conditions and hydrothermal processes, and they are widespread in both igneous and metamorphic rocks.