Hornblende , (amphibole of Hauy), a mineral species placed by Dana in the augite section of the anhydrous silicates. The chemical composition of hornblende was formerly represented by the general formula 4RO 3SiO3, in which RO may bo either calcium, magnesium, iron and sodium, or sometimes manganese and potassium; but Rammelsberg by comparing his analyses with those of others, concludes that all hornblendes are metasilicates of the general formula M2OSi02, or M2Si03. In some varieties the silica is replaced by alumina. The application of the law of isomorphism brings together under the same species many minerals that were formerly regarded as distinct; and thus aetinolite, tremolite, asbestus, and others, are now properly included in this species. In common use the name is limited, as it was formerly applied, only to the dark crystalline minerals which are met with in long slender prisms, either scattered in quartz, granite, and other igneous and metamorphic rocks, or generally disseminated throughout their mass; constituting with feldspar alone greenstone and varieties of the trappcan rocks, and also hornblende slate; with feldspar and quartz, the rock syenite, or if mica too be present, syenitic granite.
The crystals are also aggregated together to form rocks called hornblende or amphibole rocks, the texture of which is sometimes granular. The color of the mineral is usually black or dark green, owing to the presence of much iron; its hardness is 5-6; specific gravity, 3.l-3.4. It has close affinities with augite, and on cooling after fusion it has been found to assume the form and cleavage of this mineral. It appears to have been produced under conditions of fusion and cooling which cannot be imitated in the laboratory, the crystals obtained artificially being of the augite type.