Hornblende, an important rock-forming mineral, having much the same composition as augite. It is considered to be an isomorphous mixture of silicate of magnesia and lime and silicate of iron and lime, combined with an aluminous silicate of lime and magnesia.
It crystallises in monoclinic forms; has a hardness = 5.5 to 6; and specific gravity = 3.1 to 3.3. There are two tolerably well-marked varieties—viz. common hornblende and basaltic hornblende. Common hornblende is dark-green to raven-black, and is characteristic of many crystalline schists and plutonic rocks. It generally takes the form of long prismatic crystals, but is sometimes massive, fibrous, and radiating. Basaltic hornblende is generally brownish-black to pitch-black, and the crystals are usually short and well formed. It occurs as a primary constituent of many eruptive rocks. Smaragdite is a peculiar grass-green lamellar form of hornblende, characteristic of the rock Eclogite.