Pyrite cubes

Recrystallization texture is also common for chalcopyrite and sphalerite. The occurrence of chalcopyrite and sphalerite filling the interstices between the pyrite cubes and chalcopyrite inclusion within them is considered to be due to the recrystallization of pyritic ores containing chalcopyrite and sphalerite (Yui, 1983). Yui (1983) suggested that these textures found in the Besshi-type deposits are useful in interpreting ore textures of the Kuroko ores, particularly their diagenetic recrystallization features because such textures are commonly observed in the Kuroko ores (Yui, 1983 Eldridge et al., 1983). 

Sulphides. The partially ionic alkali metal sulphides Me2S have the anti-fluorite-type structure (each Me surrounded by a tetrahedron of S, and each S atom surrounded by a cube of Me). The NaCl-structure type (6/6 coordination) is adopted by several mono-sulphides (alkaline earth, rare earth metals), whereas for instance the cubic ZnS-type structure (coordination 4/4) is observed in BeS, ZnS, CdS, HgS, etc. The hexagonal NiAs-type structure, the characteristics of which are described in 7.4.2.4.2, is observed in several mono-sulphides (and mono-selenides and tellurides) of the first-row transition metals the related Cdl2 (NiAs defect-derivative) type is formed by various di-chalcogenides. Pyrite (cP 12-FeS2 type see in 7.4.3.13 its description, and a comparison with the NaCl type) and marcasite oP6-FeS2 are structural types frequently observed in several sulphides containing the S2 unit. 

The burning of pyrites is not quite so simple a matter as the burning of sulphur, as a red heat ia required, Two varieties of pyrites are Ordinarily met with. One kind, of which large quantities are to be found in Wicklow, Ireland, and is imported into this country, to a considerable extent, for the nse of sulphuric acid manufacturers, has the crystalline form of a cube, and is of a bronze-yellow color, very hard, so much so as to strike fire with steel. The following is the composition of a specimen, on the authority of R. D. Thomson—... 

Isometric (cubic) Three equal axes, at right angles to each other Cube, dodecahedron, octahedron Halite (salt), pyrite, diamond, fluorite, garnet... 

Ruthenium Sesquisulphide, RuaSs, occurs in the platinum washings of Borneo and Oregon as the rare mineral lauriie, usually in the form of minute octahedra hardness 7-5, density 6-99. It has a bright metallic lustre and contains a little osmium. Deville and Debray 5 prepared a similar sulphide artificially in the form of isometric crystals, octahedra, and cubes by heating ruthenium with pyrites and borax. 

Platinum is found in combination with arsenic as the rare mineral sperrylite, PtAs2, at Vermillion Mine in Ontario, Canada. It was first discovered by Sperry (whence its name) in 1887 along with copper and iron pyrites in contact with gold ore. It crystallises in cubes or, less frequently, in oetahedra hardness, 6-7 specific gravity, 10-602 at 20° C. It is tin-white in colour, possessed of metallic lustre, and contains traces of rhodium and antimony.4... 

Another class of sulfides of considerable importance are the disulfides, represented by FeS2, CoS2, and others. All these contain discrete S2 units with an S—S distance almost exactly equal to that to be expected for an S—S single bond. These assume one of two closely related structures. First there is the pyrite structure named after the polymorph of FeS2 that exhibits it. This structure may be visualized as a distorted NaCl structure. The Fe atoms occupy Na positions and the S2 groups are placed with their centers at the Cl positions but turned in such a way that they are not parallel to any of the cube axes. The marcasite structure is very similar but somewhat less regular. 

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.6 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. 

Pyrite and marcasite are the major minerals forming the sulfide cement, as identified by XRD and optical microscopy. These sulfides occur as both well-formed cubes and anhedral masses. Arsenic-rich areas (up to 1% by weight as estimated by EDS) occur in the pyrite and marcasite crystals as well as in iron hydroxides, but no separate arsenopyrite phase has been identified. Colloidal size (10-20 nm) iron hydroxide phases were identified using TEM. TEM-EDS analysis showed qualitative differences in arsenic, nickel, and zinc in the iron hydroxides on a nanometer scale. 

Syn. Iron Pyrites.—This a very abundant mineral, of a yellow colour and metallic lustre, crystallising in cubes or octahedrons. When heated in close vessels, it loses nearly half its sulphur, which may be collected, magnetic pyrites being left. Iron pyrites is often contaminated with arsenic, which accompanies the sulphur, and is found in the sulphuric acid made from it. 

In Figure 99, we also show photographs of some pyrite crystals. As you can see, these crystals are considerably distorted from the idealized symmetries that we have discussed. Notice also that the faces of some of the cubic crystals have striations in a direction perpendicular to the striations on neighboring faces. This is a result of the lower symmetry of the 2/m3 class, where, for example, the four-fold axes that would generate identical cube faces, are absent. 

Figure 99 Some pyrite crystals (a) cubes showing striations in different directions on adjacent faces (Huanzala, Peru) (b) cubes modified by octahedral faces (Huanzala, Peru) (c) cubes modified by dodecahedral faces (Nanisivik Mine, Northwest Territory, Canada) (d) octahedra (Huanzala, Peru).

Cobaltite (gray cube)—COBALT Pyrite (gold)—IRON Columbinite (tan. gray stripe)— NIOBIUM. TANTALUM Bismuth (shiny)... 

As a raw material for sulfur, the mineral pyrite FeS is important, but more so in ear-her times than nowadays. It is a brass-yellow mineral with such a brilliant metallic luster that it has been mistaken for gold. It commonly crystallizes in cubes. Pyrite is the most abundant of the sulfide minerals and occurs in all kinds of rocks and coal seams. Pyrite is an important ore of sulfur, less so of iron, and is sometimes mined for the associated gold and copper. In modem sulfur production, China is the only country (among the top producers of the world) whose primary sulfur source is pyrite. 

M74 Pyrite FeS2, cubic diploidal (disdodecahedral) crystals from Logrono in Spain and Kleva in Central Sweden. The striation on the cube form, meeting at right angles at the edges, is very characteristic. 

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