amphibole

Amphibole is a major group of inosilicate minerals characterized by a distinctive double-chain silicate structure. The minerals are common in metamorphic rocks such as schists and amphibolites and also occur in various igneous rocks, including diorite and andesite.

Structure and composition: Amphiboles share a general chemical formula A0-1 B2 C5 T8 O22 (OH)2. The structure consists of double chains of linked SiO4 tetrahedra, with several cation sites accommodating different elements. The A site holds large cations such as calcium, sodium, or potassium; the B site is typically calcium and other divalent cations; the C site is usually magnesium, iron (Fe2+ or Fe3+), or aluminum; the T site is mainly silicon with some aluminum. Hydroxyl groups (OH) are present in the structure.

Classification and examples: Amphiboles are often grouped as calcic (calcium-rich), sodic (sodium-rich), or aluminous, with well-known series including tremolite–actinolite, hornblende, and various magnesio- and ferro-amphiboles (e.g., anthophyllite, grunerite).

Occurrence and formation: They form over a wide range of metamorphic conditions, from low-grade to high-grade, and in igneous rocks when water-bearing magmas crystallize. The specific amphibole species stability depends on temperature, pressure, and chemical composition of the rock.

Physical properties and uses: Amphiboles typically occur as elongated prisms with colors ranging from colorless to green, brown, or black. They have a hardness around 5–6 on the Mohs scale and two good cleavage directions near 56 and 124 degrees. Some varieties, notably tremolite, actinolite, crocidolite, and amosite, can occur in an asbestiform habit, which poses health risks if fibers become airborne. Outside asbestos contexts, amphiboles are important rock-forming minerals studied in geology and petrology.

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