Blende

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 appearance but contains no lead, and was consequently often rejected as worthless. Sphalerite, introduced by E.F. Glocker in 1847, has the same meaning (σφαλερός, deceptive), and so have the miners’ terms “mock ore,” “false lead,” and “blackjack.” The term “blende” was at one time used in a generic sense, and as such enters into the construction of several old names of German origin; the species under consideration is therefore sometimes distinguished as zinc-blende.

Crystals of blende belong to that subclass of the cubic system in which there are six planes of symmetry parallel to the faces of the rhombic 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 form is the tetrahedron. Fig. 1 shows 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, whilst those of the other set are bright and smooth. A common form, shown in fig. 2, is a combination of the rhombic dodecahedron with a three-faced tetrahedron y (311); the six faces meeting in each triad axis are often rounded together into 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 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 60°.

When chemically pure, which is rarely the case, blende is colourless and transparent; usually, however, the mineral is yellow, brown or black, and often opaque, the depth of colour and degree of transparency depending on the 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 refraction high (2.369 for sodium light). The substance is usually optically isotropic, though sometimes it exhibits anomalous 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.

Fig. 2.	Fig. 3.

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 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 fine granular structure and a crystalline fracture; sometimes it occurs as a soft, white, amorphous deposit resembling artificially precipitated zinc sulphide. A compact variety of a pale liver-brown colour and forming concentric layers with a reniform surface is known in Germany as Schalenblende or Leberblende.

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 in New Jersey is known as cleiophane; snow-white crystals are also found at Nordmark in Vermland, 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 Colombia, South America) and christophite (from St Christophe mine at Breitenbrunn near Eibenstock in Saxony). Transparent blende of a red or reddish-brown colour, such as that found near Holywell in Flintshire, is known as “ruby-blende” or “ruby-zinc.” Přibramite is the name given to a cadmiferous blende from Přibram in Bohemia. Other varieties contain small amounts of mercury, tin, manganese or thallium. The elements gallium and indium were discovered in blende.

Blende occurs in metalliferous veins, often in association with galena, also with chalcopyrite, barytes, fluorspar, &c. In ore-deposits 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; for example, in nodules of clay-ironstone in the Coal Measures, in the cement-doggers of the 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 matter.

Localities for fine crystallized specimens are numerous. Mention may be made of the brilliant black crystals from Alston Moor in Cumberland, St Agnes in Cornwall and Derbyshire. Yellow crystals are found at Kapnik-Bánya, near Nagy-Bánya in Hungary. Transparent yellow cleavage masses of large size occur in limestone in the zinc mines at Picos de Europa in the province of Santander, Spain. Beautiful isolated tetrahedra of transparent yellow blende are found in the snow-white crystalline dolomite of the Binnenthal in the Valais, Switzerland.