Esfalerita

Sphalerite, or blende, is the principal zinc ore: a zinc sulphide that crystallises in the cubic system.

It is a zinc sulphide with variable chemistry: iron darkens the mineral, while trace mercury, copper or bismuth shift its colours; at Áliva cadmium imparts warm hues. The iron-rich variety marmatite is black, occurring as massive aggregates or bright crystals. It crystallises in the cubic system and may show tetrahedra and dodecahedra. At Áliva, translucent honey, amber or deep red crystals are rare; these gem-quality cleiophanes display adamantine lustre.

It commonly forms in hydrothermal veins with metallic sulphides, and is often found with quartz, calcite, dolomite or barite. At Áliva it was mined together with galena and pyrite as the zinc ore. Elmwood varieties, amber to ruby in colour with high lustre, rank among the most prized. In the Tri-State district at Joplin, well-formed crystals, often twinned and striated, range from jet-black to translucent ruby-jack on silicified limestone. At Trepča it occurs as complex, high-lustre crystals, and at Troya it is frequently associated with white dolomite.

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