Sphalerite is a mineral with a hardness of 4 out of 10 on the Mohs scale of mineral hardness [?]. These Cubicly structured gems are made of zinc sulfide, their full chemical compound being (Zn,Fe)S.
Sphalerite is a zinc iron sulfide mineral formed in an isometric system. It is trimorphous with wurtzite and matraite. All three have the same chemical compound but crystallize in three different forms.
The most common crystal forms of sphalerite are in dodecahedrons and tetrahedrons, elegant three-sided formations, often twinned, and otherwise usually massive, thus looking like brownish-black or cherry-red colonies forming a stark contrast on its host rock, for example white dolomite. It also appears as pseudo-octahedral crystals, often with rounded edges. Layered sphalerite was called "Schalenblende" in German. Crystals vary from transparent to opaque. It can occur as inclusions in the reddish-orange almandine garnet.
Sphalerite has a bright luster which is even diamond-like on cleavage surfaces. Its high refractive index nearly equals that of diamond. It emits a characteristic odor when rubbed across an unglazed porcelain surface, and when dilute hydrochloric acid is added, will produce the smell of "rotten eggs."
It forms complex ores in pegmatitic-pneumatolytic veins where crystallization occurs during hot gaseous activity in late stages of cooling. It is also found in hydrothermal veins associated with galena, marcasite, pyrite, argentite, greenockite, chalcopyrite, barite and fluorite. Pure zinc sulfide is white in powder form. As the iron content increases, the color changes from yellow to brown, red, dark green, and black (marmatite, a variety of sphalerite).
Many of the economically important sources of sphalerite are in hydrothermal areas, such as those in Oberharz, Freiberg, in the Erzebirge, in Schauinsland in the Schwarzwald (Germany); also Bleiberg in Carinthia and Pribram (Czech Republic).
Very important deposits are found in the Tri-state district in the USA (Oklahoma, Kansas, and Missouri), Sullivan (Canada), Broken Hill (Australia), Bleiberg (Austria), Yugoslavia, Raibi (eastern Alps, Italy), and Sardinia (Montevecchio, Monteponi and Campo Pisano).
Sphalerite is also found with pyrite and barite in sedimentary deposits near Meggen in Westphalia.
The most beautiful crystals come from Trepca, Yugoslavia (especially marmatites), Butte, Montana (USA), and Oruro and Potosi (Bolivia). Light-colored crystals, which are unusual for sphalerite, come from Picos de Europa, Santander, Spain. Small but fine transparent crystals in different colors are found in the metamorphic dolomites of the Binn Valley (Valais, Switzerland) and in Italy in Carrara marble. Fine marmatites occur in Bottino (Tuscany, Italy).
Sphalerite mineral is also called "black jack."
The crystal form of sphalerite has a non-metallic resinous luster. The amber-colored variety is called "honey blende" and the red variety is misnamed "ruby blende". The colorless variety is called "cleiophane".
Sphalerite has an interesting history leading to its name. Being known by the German word "Blende," it was first mentioned by Agricola in the 16th century. "Blende" was used to describe this metal-free, black, or sometimes yellow rock which, because of its heaviness and luster that looked like metal, "blendet und betrugt," "blinded and deceived" the miner into believing it had useful metal content. It was only in the 18th century that the Swedish chemist Georg Brandt discovered a zinc ore in Blende. When this happened, its original "blind-and-deceive" monicker was proven to be unfounded, but a hint of this stayed on in its present name sphalerite, from the Greek word "sphaleros" which means "deceiving."
Sphalerite crystal is often difficult to cut because of its perfect cleavage. It is often confused with other yellow gemstones and with colorless diamond. After polishing, some yellow color sphalerite does resemble a canary diamond.
As an important zinc ore, sphalerite mineral is always associated with galena. It often contains rarer metals such as cadmium, indium, gallium, germanium, gold, and silver, which are recovered during smelting.
The annual world production of zinc is about 5.8 million tons, and almost all that is derived from sphalerite.
Technical uses for zinc are in the production of brass and bronze alloys, as an anti-corrosive in the galvanizing of sheet steel and iron, galvanic elements, pigments such as zinc white, lithopone, zinc yellow, and in insecticides. Production of zinc does produce excessive residues which cause nearby rivers and soils to carry excessive levels of heavy metals, a condition which can be alleviated by effective sewage treatment.