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BLENDE, or SPHALERITE

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Originally appearing in Volume V04, Page 57 of the 1911 Encyclopedia Britannica.
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BLENDE, or SPHALERITE , a naturally occurring See also:zinc sulphide, ZnS, and an important ore of zinc. The name blende was used by G. See also:Agricola in 1546, and is from the See also:German blenden, to See also:blind, or deceive, because the See also:mineral resembles See also:lead-ore in See also:appearance but contains no lead, and was consequently oftep rejected as worthless. Sphalerite, introduced by E. F. Glocker in 1847, has the same meaning (Gr. u¢aXepos, deceptive), and, so have the miners' terms " See also:mock ore, " false lead," and " See also:black See also:jack." The See also:term " blende " was at one See also:time used in a generic sense, and as such enters into the construction of several old names of German origin; the See also:species under See also:consideration is there-fore sometimes distinguished as zincblende. Crystals of blende belong to that sub-class of the cubic See also:system in which there are six planes of symmetry parallel to FIG. I. the faces of the rhombic See also:dodecahedron and none parallel to the cubic faces; in other words, the, crystals are cubic with inclined hemihedrism, and have no centre of symmetry. The fundamental See also:form is the See also:tetrahedron. Fig. 1 shows a See also: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 See also:surface 'characters, those of one set being dull and striated, whilst those of the other set are See also:bright and smooth. A See also:common form, shown in fig.

2, is a combination of the rhombic 'dodecahedron with a three-faced tetrahedron y (311); the six faces See also:

meeting in each triad See also:axis are often rounded together into See also:low conical forms. The crystals are frequently twinned, the twin-axis coinciding with a triad axis; a rhombic dodecahedron so twinned (fig. 3) has no re-entrant angles. An important See also:character of blende is the perfect dodecahedral cleavage, there being six directions of cleavage parallel to the faces of the rhombic dodecahedron, and angles between which are 6o°. When chemically pure, which is rarely the See also:case, blende is colourless and transparent; usually, however, the mineral is yellow, See also:brown or black, and often opaque, the See also:depth of See also:colour and degree of transparency depending on the amount of See also:iron See also:present. The streak, or colour of the See also:powder, is brownish or See also:light yellow, rarely See also:white. The lustre is resinous to adamantine, and the See also:index of See also:refraction high (2.369 for See also:sodium light). The substance is usually optically isotropic, though sometimes it exhibits anomalous See also:double refraction; fibrous zinc sulphide which is doubly refracting is to be referred to the hexagonal species wurtzite. The specific gravity is 4.0, and the hardness 4. Crystals exhibit pyroelectrical characters, since they possess four uniterminal triad axes of symmetry. Crystals of blende are of very common occurrence, but owing to twinning and distortion and curvature of the faces, they are often rather complex and difficult to decipher. For this See also:reason the mineral is not always readily recognized by inspection, though the perfect dodecahedral cleavage, the adamantine lustre, and the brown streak are characters which may be relied upon.

The mineral is also frequently found massive, with a coarse or See also:

fine granular structure and a crystalline fracture; sometimes it occurs as a soft, white, amorphous See also:deposit resembling artificially precipitated zinc sulphide. A compact variety of a See also:pale See also:liver-brown colour and forming concentric layers with a reniform surface is known in See also:Germany as Schalenblende or Leberblende. A few varieties of blende are distinguished by See also:special names, these varieties depending on See also:differences in colour and chemical See also:composition.' A pure white blende from See also:Franklin in New See also:Jersey is known as cleiophane; See also:snow-white crystals are also found at Nordmark in Vermland, See also:Sweden. Black blende containing ferrous sulphide, in amounts up to 15 or 20 % isomorphously replacing zinc sulphide, is known as marmatite (from Marmato near Guayabal in See also:Colombia, See also:South See also:America) and christophite (from St Christophe mine at Breitenbrunn near See also:Eibenstock in See also:Saxony). Transparent blende of a red or reddish-brown colour, such as that found near See also:Holywell in Flintshire, is known as " See also:ruby-blende " or " ruby-zinc." Pfibramite is the name given to a cadmiferous blende from Pfibram in Bohemia. Other varieties contain small amounts of See also:mercury, See also:tin, See also:manganese or See also:thallium. The elements See also:gallium and See also:indium were discovered in blende. Blende occurs in metalliferous See also:veins, often in association with See also:galena, also with chalcopyrite, See also:barytes, fluorspar, &c. In ore-deposits containing both lead and zinc, such as those filling cavities in the limestones of the See also:north of See also:England and of See also:Missouri, the galena is usually found in the upper See also:part of the deposit, the blende not being reached until the deeper parts are worked. Blende is also found sporadically in sedimentary rocks; for example, in nodules of See also:clay-ironstone in the See also:Coal See also:Measures, in the See also:cement-doggers of the See also:Lias, and in the casts of fossil shells. It has occasionally been found on the old timbers of mines. In these cases the zinc sulphide has probably arisen from the reduction of sulphate by organic See also:matter.

Localities for fine crystallized specimens are numerous. Mention may be made of the brilliant black crystals from See also:

Alston See also:Moor in See also:Cumberland, St See also:Agnes in See also:Cornwall and See also:Derbyshire. Yellow crystals are found at Kapnik-Banya, near Nagy-Banya in See also:Hungary. Transparent yellow cleavage masses of large See also:size occur in See also:limestone in the zinc mines at Picos de See also:Europa in the See also:province of See also:Santander, See also:Spain. Beautiful isolated tetrahedra of transparent yellow blende are found in the snow-white crystalline See also:dolomite of the Binnenthal in the See also:Valais, See also:Switzerland. (L. J.

End of Article: BLENDE, or SPHALERITE

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