Disulphide Division, Pyrite group – Osometric, pyritohedral and Iron Pyrites
Pyrite, Iron Pyrites
Composition. Iron disulphide, FeS2 = Sulfur 53.4, iron 46.6. Sometimes contains small amounts of nickel cobalt and copper. Frequently carries minute quantities of gold (auriferous pyrite).
Crystallization. Isometric, pyritohedral. Most common crystal forms are the cube, the faces of which are usually striated, the striae on adjacent faces being perpendicular ton each other (Fig. 229); the octahedron, and the pentagonal dodecahedron,
Known commonly as the pyritohedron (Fig. 230). Figs. 231 to 233 show characteristic combinations of these forms. Fig. 234 shows a penetration twin that is at times observed.
Structure. Often in crystals. Also massive, granular, renfiform, globular and stalactite.
Physical Properties. Britle. H. = 6-6.5 (unusually hard for a sulphide). G. = 4.95-10. Luster metallic, splendent. Color pale brass-yellow, becoming darker at times on account of tarnish. Streak greenish or brownish black.
Tests. Easily fusible (2.5-3) to a , magnetic globule. Yields much sulfur in C. T. Gives of sulfur dioxide in O. T. or B. B. on charcoal. Insoluble in hydrochloric acid. Fine powder completely soluble in nitric acid, but may yield separated sulfur when too rapidly decomposed. Distinguished from chalcopyrite by its paler color and the fact that it cannot be scratched by steel; from gold by its being brittle.
Occurrence. Pyrite is the most common of the sulfides. It has been formed in neutral or alkaline solutions and at high temperatures (cf. marcasite, p. 167). It is a common vein mineral, uccurring in rocks of all ages and pypes, being most common in the metamorphic and sedimentary rocks. It is associated with many minerals being found most frequently with chalcopyrite, sphalerite,
Galena, etc. Is widely distributed as an accessory rock mineral in both igneous and sedimentary rocks being either primary or secondary in its origin. Frequently widel disseminated in the sedimentary rocks, also in contac methamorphic deposits. Important deposits of pyrite in the United States are in Prince William, Louisa and Pulaski counties, Virginia, where it occurs in large lenticular masses which conform in position to the foliation of the inclosing schists; in St. Lawrence County, New York; at the Davis Mine, near Charlemont, Massachusetts; in various places in California. Large deposits occurs at Rio Tinto and Other mines in Spain, also in Portugal.
Alteration. Pyrite is easily alterated to oxides of iron, usually liminote. It is, however, in general much more stable than marcasite. Pseudomorphic crystals of limestone after pyrite are common. Pyrite veins are usually capped by a cellular deposits of limonite, termed gossan. Rocks that contain pyrite are usuitable for structural purposes because the ready oxidation of the pyrite in them would serve both to disintegrate the rock and to stain it with iron oxide.
Name. the name pyrites is from a Greek word meaning fire, in allusion to the fact that when struck with steel it gives of brilliant sparks.
Use. Pyrite is often mine for the gold or copper associated with it. Because of the large amount of sulfur present in the mineral it is never used as iron ore. It is chiefly used to furnish sulfuric acid and copperas (ferrous sulphate). Sulfuric acid is perhaps the most important of all chemicals, being use for many different purposes, some of the more important being in the purification of kerosene and in the preparation of mineral fertilizers. The gas SO2 derived either though burning sulfur or by roasting pyrite is used extensively in the preparation of wood pulp for manufacture into paper. Copperas is used in dyeing, in the manufacture of inks, as a preservative of wood, and for a disinfectant.
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