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Sulfides

Sulfides include the majority of the ore minerals. They have the general formula:

XmZn

Where X = metal and Z is S, As, or Te. One cannot make generalizations about their structuresoverall. The sulfides that you need to know are pyrite, marcasite, pyrrhotite, sphalerite, galena,chalcopyrite and arsenopyrite.

Pyrite and Marcasite: (FeS2 ) Pyrite is isometric where Fe ions are bonded to covalentlybonded S2 pairs. See diagram below:

Marcasite, however, is orthorhombic. The pressure-temperature relationships are not that wellknown, however, marcasite oxidizes more easily than pyrite in the surface environment. It is notuncommon to see pyrite replacing marcasite and visa-versa.

Pyriteis easily recognized by its crystal structure and color. It usually forms striated cubes,pyritohedrons (12 pentagonal faces) and octahedrons. It is pale brass yellow. It has a hardness of6-6.5 and has a conchoidal fracture.

Occurrence: It is the most common and widespread of sulfide minerals. It is found insedimentary shales, in igneous rocks, in veins with quartz and calcite and especially as anaccessory mineral in Au deposits.

Use: It is used as an iron ore only in those countries lacking Fe-oxides. S if a by-product and isused to make sulfuric acid.

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Occurrence: Sphalerite is usually associated with galena in ore deposits. It is found in massivesulfide deposits associated in volcanic rocks associated with galena, chalcopyrite, pyrrhotite andpyrite. It is also found in carbonate sedimentary rocks with galena. These are known asMississippi Valley Type deposits.

Use: It is used to galvanize iron. It is a main constituent of brass (with Cu). It is also used inbatteries. ZnO is used in oil-based and some latex paint to prevent mildew.

Galena: is isometric and commonly occurs as cubes. It has a distinctive cubic cleavage: threedirections at right angles. Its color is lead-gray with a specific gravity of 7.5.

Occurrence: It is usually associated with sphalerite, pyrite, marcasite, minor chalcopyrite andcarbonates in Mississippi Valley Type deposits. It is also found in volcanogenic massive sulfidedeposits with sphalerite, chalcopyrite, pyrrhotite and pyrite.

Use: Lead is used in storage batteries; in pipes, shot, and as Pb sheets (radioactive shielding).PbO used to be used as filler in paints. It has been replaced by TiO2 in most cases, however,there are still the occasional Pb-based paints, or, Pb-based glazes (for ceramics).

Chalcopyrite: is tetragonal, however, crystals are rare. It usually is massive in appearance. Itis bright yellow in color and tends to tarnish. Its streak is greenish-black. Its hardness is 3.5 to 4,which distinguishes it (along with the color) from pyrite. It also alters to malachite.

Occurrence: Occurs with other sulfide minerals in massive sulfide deposits, Mississippi ValleyType deposits, and Cu-porphyry deposits.

Use: This is the main ore mineral for Cu. Cu is second in importance (after Fe) to the world forpiping and wires.

Arsenopyrite: Forms monoclinic wedge-shaped crystals, which can also be prismatic. It isbright silver-white in color with a metallic luster. It has a black streak. It is recognized by itshardness (5.5-6); its color; and its crystal shape.

Occurrence and use: commonly occurs with Sn and W minerals and also with Au mineralization.It is a source of As which is used in medicines, insecticides, and preservatives.