Hornblende: a group of minerals belonging to the class of silicates (VIII), Subclass: Inosilicates, Family; Amphiboles. Its chemical formula is (Ca,Na,K)2—3(Mg, Fe2+, Fe3+,Al)5(Si, Al)2Si6O22(OH,F)2. It is an inosilicate of Magnesium (Mg), Iron (Fe), Aluminum (Al), Calcium (Ca), possibly Sodium (Na) and Potassium (K). It crystallizes in the monocline system in prismatic crystals, often massive, granular or fibrous; color is green, brownish green, black from semitransparent to opaque. Density 3.02-3.27 g/cm3; hardness Mohs 5-6; R.I.: 1.615-1.705; 1.618-1.714; 1.632-1.730.
