BLENDE or SPHALERITE, a naturally occurring zinc sulphide, ZnS, and an important ore of zinc. The name blende was used by G. Agricola in 1546, and is from the 'German, blenden, to blind, or deceive, because the mineral resembles lead-ore in ap pearance but contains no lead, and was consequently often re jected as worthless. Sphalerite, introduced by E. F. Glocker in 1847 has the same meaning (Gr. o)aXepos, deceptive), and so have the miners' terms "mock ore," "false lead" and "black jack." Crystals of blende are cubic with inclined hemihedrism, and have no centre of symmetry. The basic form is the tetrahedron. A common form is a combination of two tetrahedra, in which the four faces of one tetrahedron are larger than the four faces of the other: further, the two sets of faces differ in surface characters : those of one set being dull and striated, while those of the other set are bright and smooth. An important character of blende is the perfect dodecahedral cleav age, there being six directions of cleavage parallel to the faces of the rhombic dodecahedron, the angles between which are 6o°.
When chemically pure, which is rarely the case, blende is col ourless and transparent ; usually, however, the mineral is yellow, brown or black, and often opaque, the depth of colour and degree of transparency depending on tree amount of iron present. The streak, or colour of the powder is brownish or light yellow, rarely white. The lustre is resinous to adamantine, and the index of re fraction high (2.369 for sodium-light) . The substance is usually optically isotropic. The specific gravity is 4•o, and the hardness 4.
A few varieties of blende are distinguished by special names, these varieties depending on differences in colour and chemical composition. A pure white blende from Franklin furnace in New Jersey is known as cleiophane. Black blende containing ferrous sulphide in amounts up to 15 or 2o% isomorphously replacing zinc sulphide, is known as marmatite (from Marmato in Colombia, South America) and christophite (from St. Christophe mine at Breitenbrunn in Saxony).
Blende occurs in metalliferous veins, often in association with galena, also with chalcopyrite, barytes, fluorspar, etc. In ore de posits containing both lead and zinc, such as those filling cavities in the limestones of the north of England and of Missouri, the galena is usually found in the upper part of the deposit, the blende not being reached until the deeper parts are worked. Blende is also found sporadically in sedimentary rocks. (L. J. S.)