HORNBLENDE, in mineralogy, a spe cks of the clay genus, of which them are four subspecies; viz. the common I the Labrador, the basaltic, and the horn blende slate. The common hornblende is of a greenish black, or raven-black, which in some varieties approaches to a greyish and even velvet black. It occurs• in mass, disseminated and crystallized. Its specific gravity is from 3.6 to 3.8. It is opaque, gives a greyish streak, is mode rately hard, and easily frangible ; it ex hales an argillaceous odour. It melts be fore the blow-pipe into a greyish black glass; the constituent parts, according to Kirwan, are, Silica . . . . 37.0 Alumina . . . 22.0 Magnesia . . . 16.0 Lime . . . . 2.0 Oxide of iron . 23.0 100.0 The common hornblende forms one of the essential ingredients of several moun tain rocks is sometimes accidentally dis seminated in others, and occurs in beds. When in beds, it is frequently accompa nied with ores, as magnetic iron-stone, iron pyrites, &c. It is found in all the northern parts of Europe ; and when pure is a capital flux for iron ores, to which purpose it is applied in Sweden, where it is obtained in large quantities.
The Labrador hornblende is found in the island of St Paul, on the coast of La brador, is usually of a brownish black, and its specific gravity is 3.S8. The horn
blende slate is of a colour intermediate between greenish and raven black ; it is massive, and is generally mixed with mica and felspar. It occurs in beds of primi tive rocks, particularly in clay slate ; also in gneiss and mica slate, and is found principally in the northern "parts of Eu rope.—The basaltic hornblende is of a velvet black, occurs almost always in sin gle imbedded crystals, which are small and middle sized. The surface is smooth and shining, except where it happens to be covered by a thin ochery crust. The specific gravity is rather less than that of the hornblende slate. It melts before the blow-pipe into a brownish black glass. Bergman has analyzed it, and found it contained.
Silica . . . . 58.0 Alumina . . . 27.0 Iron . . . . 9,0 Lime . . • . 4.0 Magnesia . . . 1.0 [99.0 Loss . . . 1.0 100.0 It is found in Saxony, Bohemia, Scot land, Italy, &c. It resists decomposition* longer than basalt, hence crystals of it are found in clay, formed by the decomposi tion of basaltic rock. Common hornblende is difficultly frangible, but the basaltic is very easily frangible.