A mineral, β-ZnS, also called blende. It is the low-temperature form and more common polymorph of ZnS. Pure β-ZnS on heating inverts to wurtzite, α-ZnS, at 1020°C (1868°F), but this temperature can be lowered substantially by impurity-atom solid solution (especially Cd²⁺ and Fe²⁺) and sulphur fugacity. Sphalerite crystallises in the hextetrahedral class of the isometric system with a structure similar to that of diamond. The space group is F43m, and the cubic unit cell has an edge a = 0.543 nanometer, which contains four ZnS molecules. Zinc atoms occupy the positions of half the carbon atoms of diamond, and sulphur atoms occupy the other half. Each zinc atom is bonded to four sulphur atoms, and each sulphur atom is bonded to four zinc atoms. The common crystal forms of sphalerite are the tetrahedron, dodecahedron, and cube, but crystals are frequently complex and twinned (see illus.). The mineral is most commonly in coarse to fine, granular, cleavable masses. The lustre is resinous to submetallic; the colour is white when pure, but is commonly yellow, brown, or black, darkening with increased percentage of iron. It has been shown that excess sulphur can also contribute to the darkening of the color. There is perfect dodecahedral cleavage; the hardness is 3½ on Mohs scale; specific gravity is 4.1 for pure sphalerite but decreases with increasing iron content.