Economic Features
The principal use of pyrite is in the manufacture of sulphuric acid. Large quantities of acid are used in the manufacture of fertilizers from phosphate rock, and during war times in the manufacture of munitions. Sulphuric acid converts the phosphate rock into superphosphate, which is soluble and available for plant use. Other uses of the acid are referred to in connection with sulphur. Pyrite is also used in Europe for the manufacture of paper from wood-pulp, but in the United States native sulphur has thus far been exclusively used for this purpose. The residue from the roasting of pyrite is a high-grade iron ore material frequently very low in phosphorus, which is desirable in making up mixtures for iron blast furnaces.
Most of the countries of Europe are producers of pyrite, and important amounts are also produced in the United States and Canada. The European production is marketed mainly on that continent, but considerable amounts come to the United States from Spain.
Before the war domestic sources supplied a fourth to a third of the domestic demand for pyrite. Imports came mainly from Spain and Portugal to consuming centers on the Atlantic seaboard. The curtailment of overseas imports of pyrite during the war increased domestic production by about a third and resulted also in drawing more heavily on Canadian supplies, but the total was not sufficient to meet the demand. The demand was met by the increased use of sulphur from domestic deposits (p. 109). At the close of the war supplies of pyrite had been accumulated to such an extent that, with the prospect of reopening of Spanish importation, pyrite production in the United States practically ceased. War experience has demonstrated the possibility of substitution of sulphur, which the United States has in large and cheaply mined quantities. The future of the pyrite industry in the United States therefore looks cloudy, except for supplies used locally, as in the territory tributary to the Great Lakes, and except for small amounts locally recovered as by-products in the mining of coal or from ores of zinc, lead, and copper. Pyrite production in the past has been chiefly in the Appalachian region, particularly in Virginia and New York, and in California.
Geologic Features
Pyrite, the yellow iron sulphide, is the commonest and most abundant of the metallic sulphides. It is formed under a large variety of conditions and associations. Marcasite and pyrrhotite, other iron sulphide minerals, are frequently found with pyrite and are used for the same purposes.
The great deposits of Rio Tinto, Spain, which produce about half of the world's pyrite, were formed by replacement of slates by heated solutions from nearby igneous rocks. The ores are in lenticular bodies, and consist of almost massive pyrite with a small amount of quartz and scattered grains and threads of chalcopyrite (copper-iron sulphide). They carry about 50 per cent of sulphur, and the larger part carries about 2 per cent of copper which is also recovered.
Similar occurrences of pyrite on a smaller scale are known in many places. Pyrite is very commonly found in vein and replacement deposits of gold, silver, copper, lead, and zinc. In the Mississippi valley it is extracted as a by-product from the lead and zinc ores, and in the Cordilleran region large quantities of by-product pyrite could easily be produced if there were a local demand. The pyrite deposits of the Appalachian region are chiefly lenses in schists; they are of uncertain origin though some are believed to have been formed by replacement of metamorphosed limestones and schists.
Under weathering conditions pyrite oxidizes, the sulphur forming sulphuric acid,—an important agent in the secondary enrichment of copper and other sulphides,—and the iron forming the minerals hematite and limonite in the shape of a "gossan" or "iron-cap."
Pyrite is likewise frequently found in sediments, apparently being formed mainly by the reducing action of organic matter on iron salts in solution. In Illinois and adjacent states it is obtained as a by-product of coal mining.