Sphalerite, Stibnite, & Pentlandite

SPHALERITE - ZnS (zinc iron sulfide)
Sphalerite is sulfide mineral in the isometric crystal system which crystallizes in the -43m crystal class. Its characteristic elements are zinc and sulfur in an equal ratio, although over forty percent of the zinc may be replaced by iron (II), with more iron present if the mineral was formed at higher temperatures. Small amounts of cadmium and manganese may be present as well. Sphalerite is most commonly found as tetrahedral crystals, although they may also be cubes or dodecahedrons. These crystals may form either granular, cleavable masses, or encrusting aggregates over the surface of another mineral. Sphalerite may range in color from pale yellow, light brown, or white to brown, red, green, or black, with darker colors representing a high iron content. The same principle applies to the streak of the mineral, which may be anywhere from white to light or medium brown. The luster of sphalerite also ranges over a spectrum, from resinous to adamantine to submetallic. In thin section, sphalerite appears colorless, pale yellow, or pale brown. It is not pleiochroic, but does display high optical relief. The crystals may contain simple or complex twins on one face, as well as perfect dodecahdral cleavage in six directions. They will fracture conchoidally, as well as behaving in a brittle manner when put under strain.

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An example of the sulfide mineral sphalerite.	A diagram of the -43m crystal class.

Sphalerite is sometimes referred to as zincblende, and has been known as blackjack to the miners who sought it for the elements zinc and cadmium. It is found commonly in hydrothermal sulfide deposits with other sulfide minerals, as well as quartz, carbonates, and sulfates.

STIBNITE - Sb2S3 (antimony sulfide)
Stibnite is a less common sulfide mineral. It is composed of antimony and sulfur in a two to three ratio, although it may sometimes include gold, silver, iron, lead, or copper as well. Stibnite is in the orthorhombic crystal system, in the class 2/m2/m2/m. The massive or coarse to fine granular crystals may be observed as blades made up of radiating crystal groups, forming a beautifully distinctive mineral. It is a dark lead gray to black in color which may have an iridescent tarnish to it, along with a metallic luster. Stibnite's streak is also a dark lead gray. Twinning is present on two faces, and striations may also be present on one face. Cleavage may be observed in three directions, one of which is perfect. Stibnite is an isotropic mineral.

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An example of the sulfide mineral stibnite.	A diagram of the 2/m2/m2/m crystal class.

Stibnite is formed in low temperature hydrothermal veins or replacement deposits, where it is later mined as the chief ore of atimony. It is commonly found with other sulfides and minerals such as galena, cinnabar, sphalerite, barite, realgar, orpiment, and gold.

PENTLANDITE - (Fe, Ni)9S8 (iron nickel sulfide)
Pentlandite is another less common sulfide mineral. It is part of the isometric crystal system and the 4/m-32/m crystal class. Pentlandite may contain either iron or nickel and sulfur in a ration of nine to eight, with iron and nickel equally as common in crystals. Cobalt may sometimes also be found substituting for either of the two. This mineral has a metallic luster and has a yellow-ish bronze color, and a bronze, brown streak as well. Pentlandite is an isotropic mineral and is therefore opaque in hand sample and thin sections. Crystals of pentlandite are rare, but may occasionally be found in massive, granular aggregates with octahedral parting in one crystal direction. Cleavage is completely absent, but crystals will still behave in a brittle manner under pressure.

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An example of the sulfide mineral pentlandite.	A diagram of the 4/m-32/m crystal class.

Pentlandite may be found in igneous intrusions with other nickel minerals, all of which are mined as an ore of that element. It is often associated with pyrrhotite and chalcopyrite, other sulfide minerals.