hornblende

(hérn’blend), n. [ horn, = E. horn, + blende, > E. blende: see blende.} A common mineral, crystallizing in the monoclinic system with a prismatic angle of 1244°. Parallel to this, the fundamental prism, it has perfect cleavage. It occurs usually in ma ive forms, varying in structure from compact to columnar and fibrous, with the fibers parallel or curved, and also, but le often, lamellar. In composition it varies widely, from the white tremolite, a silicate of calcium and magnesium, to the green actinolite, which contains also more or le iron, and to the dark-green, brown, and black varieties, pargasite and common hornblende, which contain alumina as well as lime, magnesia, and iron; manganese is also present in some varieties, and le commonly sodium and pota ium. Asbestos, mountain-cork, and mountain-leather are included here, and nephrite or jade is a tough, compact variety. Hornblende is a constituent of many crystalline rocks, as syenite, diorite, hornblende schist, some kinds of trachyte, andesite, etc. The name amphibole is often used as the general term to include all the varieties? The hornblende or amphibole group of minerals includes also the related orthorhombic species anthophyllite, and the monoclinic arfvedsonite, crocidolite, glaucophane, etc. In geology, hornblende or hornblendic is often prefixed to names of rocks to indicate the accidental presence in greater or le quantity of that mineral, in addition to the other ingredients which the rock usually contains. Hornblende is a frequent result of the metamorphism of other minerals, especially of augite.—Hornblende andesite. See andesite.—Hornblende basalt, a very basic rock of holocrystalline texture, somewhat porphyritic, containing a comparatively small amount of feldspar and scattered crystals of hornblende. Itis a characteristic rock of the Rhongebirge in Bavaria, etc.—Hornblende gabbro, a variety of gabbro in which the diallage is more or le replaced by hornblende.— Hornblende rock, a rock consisting chiefly of black or dark-green hornblende, but often interlaminated with feldspar, quartz, or mica. Also called amphibolite. The same rock when it has a schistose character is called hornblende slate or hornblende schist.— Labrador hornblende, See hypersthene.