Pyrites

P5Trite and calcite are mineral crystals that represent a wide Habitus and Tracht variation. Pyrite is the most persistent mineral among sulfide minerals, occurring in a wide range of modes, including inorganic processes and bacterial action, and it can also be synthesized by hydrothermal or chemical vapor transport methods. 

P5U ite crystals exhibit a wide range of Tracht and Habitus, and also occur in unusual forms of polycrystalline aggregate, such as framboidal pyrite. Although numerous crystal faces have been reported, the most important ones are 100, 111, and 210. Calcite also exhibits a variety of Tracht and Habitus, such as platy, nail-head, prismatic, or dog-tooth forms, but 1011 is the only F face. In this chapter, we focus our attention on the factors controlling the observed variations in Tracht and Habitus of p3U ite and calcite. 

Pyrite is the most common mineral among sulfides. It occurs not only as a major mineral of sulfide ore deposits of base metals, such as Cu, Pb, Zn, in vein-t) e, massive-replacement t) e, kuroko-t5q3e deposits, etc., but also sporadically as an accessory mineral in volcanic, sedimentary, and metamorphic rocks. It also occurs as a precipitate in hot springs, and it may be formed by bacterial action. Pyrite itself is not an ore of Fe, though it contains iron, and at best may have economic value as an ore to obtain sulfuric acid. However, due to its occurrence in and 

In the crystal structure of pyrite (Fig. 11.1), molecules with dumbbell form and 

The surface micro topographs of the three major faces of natural pyrite crystals have the following characteristics (see Fig. 11.2 for a schematic representation). 

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The first crystal structure to be detennined that had an adjustable position parameter was that of pyrite, FeS2 In this structure the iron atoms are at the comers and the face centres, but the sulphur atoms are further away than in zincblende along a different tln-eefold synnnetry axis for each of the four iron atoms, which makes the unit cell primitive. 

Unfortimately for modem crystallographers, all of tlie crystal stmctiires that could be solved by the choose-the-best-of-a-small-niunber-of-possibilities procedure had been solved by 1920. Bragg has been quoted as saying that the pyrite stmcture was very complicated , but he wrote, in about 1930, It must be realized, however, that (cases having one or two parameters) are still extremely simple. The more typical crystal may have ten, twenty, or forty parameters, to all of which values must be assigned before the analysis of the stmcture is complete. This statement is read with amusement by a modem crystallographer, who routinely works with hundreds and frequently with thousands of parameters. 

Each of these elements occurs naturally as a sulphide ore arsenic as realgar As S,, orpiment As, Sg and arsenical pyrites with approximate formula FeAsS antimony as stibnite Sb2S3 and bismuth as B12S3. 

Large deposits of free sulphur occur in America, Sicily and Japan. Combined sulphur occurs as sulphides, for example galena, PbS, zinc blende, ZnS, and iron pyrites, FeSj, and as sulphates, notably as gypsum or anhydrite, CaS04. 

Many of these sulphides occur naturally, for example iron(ll) sulphide, FeS (magnetic pyrites), and antimony(III) sulphide, Sb S, (stibnite). They can usually be prepared by the direct combination of the elements, effected by heating, but this rarely produces a pure stoichiometric compound and the product often contains a slight excess of the metal, or of sulphur. 

After aluminium, iron is the most abundant metal and the fourth most abundant of all the elements it occurs chiefly as oxides (for example haematite (FCjO,), magnetite (lodestonej (FC3O4) and as iron pyrites FeSj- Free iron is found in meteorites, and it is probable that primitive man used this source of iron for tools and weapons. The extraction of iron began several thousand years ago, and it is still the most important metal in everyday life because of its abundance and cheapness, and its ability to be cast, drawn and forged for a variety of uses. 

Sulfur occurs native in the vicinity of volcanos and hot springs. It is widely distributed in nature as iron pyrites, galena, sphalerite, cinnabar, stibnite, gypsum, epsom salts, celestite, barite, etc. 

Sanskrit Jval Anglo-Saxon gold L. aurum, gold) Known and highly valued from earliest times, gold is found in nature as the free metal and in tellurides it is very widely distributed and is almost always associated with quartz or pyrite. 

Thallium occurs in crooksite, lorandite, and hutchinsonite. It is also present in pyrites and is recovered from the roasting of this ore in connection with the production of sulfuric acid. It is also obtained from the smelting of lead and zinc ores. Extraction is somewhat complex and depends on the source of the thallium. Manganese nodules, found on the ocean floor, contain thallium. 

Prussic acid, see Hydrogen cyanide Pyrite, see Iron disulfide Pyrochroite, see Manganese(H) hydroxide Pyrohytpophosphite, see diphosphate(IV)... 

Two methods have been proposed for the analysis of sulfur in impure samples of pyrite, EeS2. Sulfur can be determined in a direct analysis by oxidizing it to S04 and precipitating as BaS04. An indirect analysis is also possible if the iron is precipitated as Ee(OH)3 and isolated as Ee203. Which of these methods will provide a more sensitive determination for sulfur What other factors should be considered in deciding between these methods ... 

A sample of impure pyrite known to be approximately 90-95% w/w EeS2 is to be analyzed by oxidizing the sulfur to S04 and precipitating as BaS04. How many grams of the sample must be taken if a minimum of 1 g of BaS04 is desired ... 

Total 1991 world production of sulfur in all forms was 55.6 x 10 t. The largest proportion of this production (41.7%) was obtained by removal of sulfur compounds from petroleum and natural gas (see Sulfurremoval and recovery). Deep mining of elemental sulfur deposits by the Frasch hot water process accounted for 16.9% of world production mining of elemental deposits by other methods accounted for 5.0%. Sulfur was also produced by roasting iron pyrites (17.6%) and as a by-product of the smelting of nonferrous ores (14.0%). The remaining 4.8% was produced from unspecified sources. 

World resources of sulfur have been summarized (110,111). Sources, ie, elemental deposits, natural gas, petroleum, pyrites, and nonferrous sulfides are expected to last only to the end of the twenty-first century at the world consumption rate of 55.6 x 10 t/yr of the 1990s. However, vast additional resources of sulfur, in the form of gypsum, could provide much further extension but would require high energy consumption for processing. 

Fig. 14. Coal flotation flow sheet suggested for increased sulfur removal (29). Pyritic sulfur removal from coal makes it imperative to closely control pulp...

Roasting of sulfide and sulfate ores (ZnS, pyrites, CU2S, CuCoS, nickel sulfides)... 

Table 8 shows by example the problem of batch homogenization, where 1.4 kg pyrites and 4 kg salt must be uniformly dispersed throughout a 1000 kg batch, nearly one-third of which is 300 p.m (50 mesh) sand and nearly one-half of which is cuUet (1—2 cm) glass. 

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