Sulphides

Iron pyrites, pyrite, marquisite, or mundic is the name given to a brass-coloured sulphide of iron to which the formula FeS₂ is usually ascribed. It often occurs in coal, and is known by miners as " brass " or " fool's gold." The spontaneous combustion of coal is frequently facilitated by the oxidation of iron pyrites to iron sulphate. It often crystallises in cubes, the faces showing striations; but a very characteristic form is the pentagonal dodecahedron, usually termed the pyritohedron. Hardness 6 to 6.5; density 4.8 to 5.2. It leaves a greenish or brownish-black streak.

Peruvian pyrites is called stone of the Incas, because the ancient sovereigns of Peru attributed great virtues to it. Rings, amulets, mirrors, etc., were made of it. A century ago iron pyrites was used in this country to a considerable extent for feminine ornaments.

Pyrites occurs in rocks of all ages, both eruptive and sedimentary. In some cases it is evidently an original mineral, whilst in others it has been formed as a secondary product. Large quantities are found in Spain and are used in the manufacture of sulphuric acid, ferrous sulphate, and ferric oxide.

Iron pyrites is found in masses in the " twenty-inch " coal seam of the Forest of Dean, the masses being locally called " dogs." It is common in coal measures generally, having probably been formed by the reducing action of organic matter on waters charged with iron sulphate in solution. Explosive pyrites is the name given to a variety of pyrites found in Cork, which decrepitates violently upon application of gentle heat. The decrepitation is due to the presence of carbon dioxide under high pressures confined in minute cavities in the mineral. Decrepitation usually begins at about 30° to 35° C., and it has been suggested that the explosions which sometimes occur in domestic grates may be due to the presence in the coal of this variety of pyrites. Frequently metals other than iron are associated with pyrites, chief among which are copper, nickel, cobalt, tin, gold, thallium, and silver.

When oxidised by weathering, the final product of pyrites is hydrated ferric oxide or rust, as stated above, but the method of oxidation depends upon circumstances. In the absence of carbon dioxide or carbonates, it is usual for oxidation to ferrous sulphate to first take place; whence the mineral copperas or melanterite (see below). This, in course of time, oxidises to limonite. In the presence of carbon dioxide, however, it appears that the sulphate is not first formed, but the readily oxidisable carbonate, which is then converted into limonite.

The presence of pyrites in mineral veins is frequently betrayed by the brown, rusty appearance of the upper parts of the deposits. This is well exemplified in the case of certain copper lodes, the iron of the contained copper pyrites having become converted into hydrated ferric oxide or rust. Such weathered zones are known as gozzans, and frequently present a cavernous or honeycombed appearance, due to the removal of pyritic material. In France the gozzan is termed the chapeau de fer, and in Germany der eiserne Hut, and an old adage states that

" A Lode that wears no iron hat Is never likely to be fat."

Marcasite or white iron pyrites is the name given to a rhombic variety of pyrites which occurs in concretions, known locally as " fairy balls " and " thunderbolts." Its crystallographic elements are

a: b: с = 0.7662: 1: 1.2342.

The exterior of the nodules is often brownish, but when broken open they exhibit a radiating structure, usually pale yellow in colour, although pure mareasite is probably tin-white in appearance. It rapidly tarnishes, and it is difficult to keep a fracture bright for museum purposes. Concretions of mareasite are found in chalk and in many clays. Hardness 6 to 6.5; density 4.7 to 4.8.

Owing to its jagged outline mareasite is frequently known as cockscomb pyrites and spear pyrites.

Cobalt-nickel pyrites, (Fe, Co, Ni)S₂, has been found in Westphalia as small cubic crystals, steel-grey in colour, giving a greyish-black streak. Density 4.7; hardness 5 to 5.5. Iron nickel pyrites, (Fe,Ni)S₂, occurs in Norway and in the Sudbury district, Ontario.

Pyrrhotite or magnetic pyrites, Fe₇S₈ or 5FeS.Fe₂S₃, is slightly redder than ordinary pyrites, from which, however, it may be readily distinguished, both on account of its magnetic properties and by its greater softness. Hardness 3.5 to 4.5; density 4.4 to 4.7. It occurs in Cornwall, in the lavas of Vesuvius, and in various other parts of Europe; in Canada and the U.S.A. Two varieties are known: namely α-pyrrhotite, which is rhombic; and β-pyrrhotite, which is hexagonal.

Troilite, FeS, is brownish in colour, and occurs in nodules in most meteorites containing iron. Hardness 4; density 4.7 to 4.8.

Copper pyrites or chalcopyrite, Cu₂S.Fe₂S₃, is a brass-coloured sulphide which crystallises in the tetragonal system, but is isomorphous with pyrites, since its crystallographic elements are

a: с = 1: 0.9853.

It yields a greenish-black streak; hardness 3.5 to 4; density 4.1 to 4.3. It should contain some 34.6 per cent, of copper and 30.5 per cent, of iron; but the percentage of iron is frequently higher, due probably to admixture of iron pyrites. Copper pyrites constitutes the most important copper ore in Cornwall. It is found also in Scotland, Sweden, and many other parts of Europe, in Australia, and the U.S.A. Barnhardtite, 2Cu₂S.Fe₂S₃, resembles the previous ore in appearance, and receives its name from the place of its discovery, namely, Barnhardt's Land. A varying mixture of sulphides of iron and copper is present in the mineral known variously as erubescite, Bornite, and horseflesh ore. The name Bornite was given in honour of Ignatius von Born, who had charge of the mineral collection in Vienna. It is not a happy name, however, since it is liable to be confused with bournonite, a thio-antimonite of copper and lead. Its copper content ranges from 50 to 70 per cent., and its iron from 6 to 20 per cent. When fresh its fracture is reddish brown, but it tarnishes rapidly, yielding peacock colours. Crystalline forms (cubic) occur in Cornwall, chiefly at the Carn Brea, Tincroft, and Cook's Kitchen mines near Redruth.

Hardness 3; density 4.4 to 5.5. The usually accepted formula, Cu₃FeS₃, was based on analyses of Cornish crystals, but the Canadian mineral corresponds more closely to Cu₅FeS₄.

In this connection it is interesting to note that Roman coins and other bronze objects which were thrown into certain thermal springs as votive offerings and have since been examined are found to have become partially converted into various sulphides of copper, including chalcopyrite and erubescite.

A double sulphide of copper and iron known as Chalmersite, CuFe₂S₃, occurs in the ore deposits of the Prince of Wales Sound, Alaska, and in small quantities in Brazil. It is massive, pale yellow in colour, and strongly magnetic, which latter property enables it to be readily separated from the copper pyrites with which it is intimately associated. Density 4.04 to 4.68. The crystals are rhombic,

a: b: c = 0.5734: 1: 0.9649,

and isomorphous with copper glance.

Cubanite, CuS.Fe₂S₃, is a bronze-coloured mineral found in Cuba, whence its name. Hardness 4, density 4.0 to 4.2.

A black variety of blende, occurring in Cornwall and elsewhere, is known as marmatite, and has the composition corresponding to 4ZnS.FeS.

Pentlandite, a double sulphide of iron and nickel approximating to 2FeS.NiS, occurs as cubic, bronze-coloured crystals in Argyllshire, Inveraray, and Cornwall. Hardness 3.5 to 4; density 4.6.

Arsenical pyrites or mispickel, FeS₂.FeAs₂, containing 34 per cent, of iron, occurs in rhombic crystals, with frequent twinning, and of the following parameters: -

a: b: с = 1.7588: 1: 1.4793.

It is a grey mineral; hardness 5.5 to 6; density 6 to 6.4. It is found in Devon and Cornwall (where it is worked on a small scale), Silesia, Norway, Sweden, and U.S.A., and constitutes an important source of arsenic. If cobalt is also present the mineral is known as glaucodote. When struck with a hammer, mispickel possesses the characteristic odour of arsenic and is hence known by Cornish miners as arsenical mundic or simply as arsenic. The ore readily oxidises, and when weathered gives rise to arsenates such as scorodite, etc.

Pacite consists of rhombic crystals of FeS₂.4FeAs₂.

A double sulphide of iron and chromium, FeS.Cr₂S₃, occurs as the rare mineral daubreelite associated with troilite. It possesses a brilliant metallic lustre, is black in colour, but not magnetic. Density 5.0.