The Sulphides are complex minerals in both chemistry and origin. They are usually deposited from hot aqueous solutions in fault zones of the crust, located near large igneous bodies called batholiths. Great effort has been made to understand the origin, formation and properties of Sulphides because they are amongst the most important industrial minerals. Their compounds give rise to most metals used by society.

The Sulphides are in oxygen-free compounds of Sulphur and one or more metals. In the simplest form the Sulphide structure forms spherical packing of large Sulphur atoms with the smaller metal atoms in the spaces between. The chemical bonding between metal and Sulphur varies in degrees of metallic, ionic and covalent bonds. The structural unit often forms a cube and consequently many Sulphides take on a cubic or octahedral crystal habit.

Most Sulphides have a strong metallic appearance, with a strong colour and metallic lustre. The majority are opaque, have high densities and many of them are brittle, which easily distinguishes them from the native metals. Few stand up well to atmospheric weathering and usually convert to secondary minerals.

Sulphides may be subdivided into the simple compounds of metal with Sulphur, the Sulphides, and into compounds of a metal with Sulphur plus a semi-metal such as Arsenic, Antimony or Bismuth. These are known as Sulfosalts. Examples include Enargite and Bournonite.

Usually included in this group are the corrsponding, but much rarer compounds in which the Sulphur is replaced entirely by Selenium, Tellurium, Antimony or Bismuth. Minerals belonging to the selenide, telluride, antimonide and arsenide subclasses tend to have very similar properties to each other and to the Sulphides. This makes individual identification difficult.

The main mineral groups of the Sulphide class are:

• Compounds with semi-metals - the sulphur is directly bonded to the semi-metal with no other metallic elements present. Examples include Realgar AsS and Orpiment As2S3. Other closely related species contain a heavy transition metal as in Cinnabar HgS and Greenockite CdS.
• Stibnite Group - includes two minerals with similar structures and crystals, although the constituent metals are Antimony and Bisnmuth respectively. Both metals however are similar chemically which translates into the striking similarity of Stibnite and Bismuthinite. The related mineral Guanajuatite Bi2Se3 is likewise similar because selenium is chemically similar to sulphur. Yet all three minerals are found in very different deposits and are not equally common.
• Molybdenite Group - consits of two sulphides that have a layered structure resembling graphite, but are much denser and less dark. The layers are held together by weak van der Waals forces and are not directly bonded. Within these layers each molybdenum or tungsten atom is surrounded by six sulphur atoms, at the corners of a trigonal prism. Each sulphur, in turn, is surrounded by three metal atoms. The two minerals are similar in appearance and properties, but tungstenite is a little harder and much heavier. Related minerals include Dyscrasite and Domeykite.
• Galena Group - consists of several minerals including sulphides, tellurides and selenides that have a halite-like structure with each metallic atom surrounded by six nonmetallic atoms and vice versa. Galena, Altaite and Clausthalite are typical and illustrate the chemical similarity of the three non metals they contain, namely sulphur, tellurium and selenium respectively. The sulphides Alabandite MnS and Oldhamite CaS are closely related but are rare because Mn is more stable as the oxide and Ca is more stable in silicate and carbonate structures.
• Argentite Group - consists of sulphides, selenides and tellurides of silver, copper and gold. Its members are closely related the Chalcocite group and are important ores of silver and copper. The Argenitie minerals are all cubic or octahedral. They have metallic appearance and similar properties but differ mainly in cleavage, brittleness and colour. Their have widely differing densities, the gold-bearing minerlas being the heaviest.
• Chalcocite Group - orthorhombic sulphides that appear to be in a solid solution with Argentite. However the full extent of the series is not certain. Members include Chalcocite Cu2S, Stromeyerite AgCuS and Sternbergite AgFeS, ending in Acanthite Ag2S. Two minor minerals include Crookesite and Eucairite.
• Sphalerite Group - several minerals, including Sphalerite, that are similar in structure, crystal form, cleavage and chemical properties. All occur in similar environments, notably sulphide-bearing hydrothermal veins. They have a diamond-like structure with half the carbon sites occupied by sulphur, selenium or tellurium, and the other sites taken up by zinc, mercury, copperm iron, tin or a mixture of these metals.
• Niccolite Group - includes Niccolite, Pyrrhotite and Breithauptite. All have hexagonal structure in which each metal atom is surrounded by six nonmetal atoms, each of which in turn is surrounded by six metal atoms at the corners of a trigonal prism. Other closely related minerals include Covellite, Millerite, Pentlandite abd Klockmannite.
• Pyrite Group - consits of very different minerals but all have a cubic structure. In Pyrite an Iron atom occupies each corner of a cube and the middle of each face. A pair of Sulphur atoms is mid way along each edge. The packing of atoms is known as face-centred cubic. Because the bonds between the metal and sulphur are largely covalent the minerals are brittle. However they are amongst the hardest. Other species include Marcasite, Arsenopyrite and Safflorite.
• Cobaltite Group - includes three minerals with a structure close to that of Pyrite. However, in addition to Sulphur these also have partial Arsenic or Antimony substitution. All three are major ores of Cobalt but the amount of metal is variable and nickel and iron can also be present. Occasionalyy the As is substituted by Sb or Bi. Colour is usually used to distingusih these minerals from other sulphides but because their appearance is similar and their properties are variable due to the varying composition, they are difficult to tell apart from each other.
• Skutterudite Group - contains a solid arsenide solution series ranging from Smaltite (Co,Ni)As to Chloanthite (Ni,Co)As. Any intermediate member is usually referred to as Skutterudite. These minerals have a deficiency in As. They are a major ore of Cobalt and Nickel and occur in medium-temperature hydrothermal veins. The crystal structure is similar to Pyrite.
• Krennerite Group - includes the tellurides Calaverite, Sylvanite and Krennarite. They are found in low-temperature veins with other tellurides, gold and sulphides. Krennerite and Calaverite have the same chemical composition but the former crystallises in the orthorhombic system and is more complex. Sylvanite is probably isostructural with Calaverite but half the metal sites are occupied by silver, not gold. Both crystallise in the monoclinic system.
• Pyrargyrite Group - contains two silver minerals, Pyrargyrite Ag3SbS3 and Proustite Ag3AsS3. Both occur in low-temperature veins and are deep red making identification difficult. There is little evidence for a solid solution series between the two.
• Tetrahedrite Group - is a complete solid solution series ranging from Tetrahedrite (Sb end-member) to Tennantite (As end-member). Copper is the mainmetal in each but other minerals, including iron and zinc, substitute extensively. Minerals in this group are amongst the commonest sulphosalts. Their metallic grey appearance, brittlness and highly characteristic tetrahedral crystals (hence the name) make them easy to identify. However they are not easily distinguished individually without complex tests.
• Enargite Group - contains Enargite, an As-rich copper sulphide and Famatinite an Sb-rich analogue. However the two minerals can exchange some of their nonmetallic elements so that Enargite can contain up to 6% of SB and Famatinite up to 10% As in place of Sb. both occur together in medium temperature hydrothermal veins.
• Bournonite Group - consists of two lead-copper sulphides. Bournonite can contain appreciable amounts of Sb and Seligmannite can be rich in Arsenic. There is an incomplete solid solution between them. Although rarely found together, both occur in medium temperature hydrothermal veins with other sulfosalts and sulphides.