HORNBLENDE

hornblende, an important member of the amphibole group of rock-forming minerals. the name is an old one of german origin, and was used for any dark-coloured prismatic crystals from which metals could not be extracted. it is now applied to the dark-coloured aluminous members of the monoclinic amphiboles, occupying in this group the same position that augite occupies in the pyroxene group. the monoclinic crystals are prismatic in habit with a six-sided cross-section; the angle between the prism-faces (m), parallel to which there are perfect cleavages, is 55 deg. 49'. the colour (green, brown or black) and the specific gravity (3.0-3.3) vary with the amount of iron present. the pleochroism is always strong, and the angle of optical extinction on the plane of symmetry (x in the figure) varies from 0 deg. to 37 deg. the chemical composition is expressed by mixtures in varying proportions of the molecules ca(mg, fe)3(sio3)4, (mg, fe)(al, fe)2sio6 and naal(sio3)2. numerous varieties have been distinguished by special names: edenite, from edenville in new york, is a pale-coloured aluminous amphibole containing little iron; pargasite, from pargas near abo in finland, a green or bluish-green variety; common hornblende includes the greenish-black and black kinds containing more iron. the dark-coloured porphyritic crystals of basalts are known as basaltic hornblende. [illustration.] hornblende occurs as an essential constituent of many kinds of igneous rocks, such as hornblende-granite, syenite, diorite, hornblende-andesite, basalt, &c.; and in many crystalline schists, for example, amphibolite and hornblende-schist which are composed almost entirely of this mineral. well-crystallized specimens are met with at many localities, for example: brilliant black crystals (syntagmatite) with augite and mica in the sanidine bombs of monte somma, vesuvius; large crystals at arendal in norway, and at several places in the state of new york; isolated crystals from the basalts of bohemia. (l. j. s.)