Page:EB1911 - Volume 22.djvu/708

Rh PYRITES, a term applied to iron disulphide when crystallized in the cubic system, but used also in a general sense to designate a group of metallic sulphides of which this mineral is the most characteristic example. When employed as a group-name the constituent species are distinguished by prefixes: thus the type is called iron pyrites, whilst other species are known as copper pyrites, arsenical pyrites, &c. The original word pyrites (from Gr. vriip, fire) had reference to the fact that sparks might be elicited on striking the mineral violently, as with flint, so that vrvplrrys Mfios meant a stone which struck fire. Hence the name seems to have been applied also to iiint, and perhaps to emery and other hard stones. Nodules of pyrites have been found in prehistoric barrows and elsewhere under conditions suggesting their use as a primitive means of producing fire. Even in late historic time it was employed in some of the old wheel-lock guns. Iron-pyrites was formerly called marcasite, a word variously written marcasin, marchasite, marchesite, marque site, &c. The two names are now applied to distinct mineral species. The compound FeS2 is dimorphous, and the modern practice is to distinguish the cubic forms as pyrites and the orthorhombic as marcasite (q.v.). Sometimes, however, the term pyrites is loosely applied to both species, and the cubic pyrites is then differentiated by the name “ pyrite ”-a form which brings the last syllable into harmony with the spelling of the names of most minerals. Iron pyrites, or pyrite, belongs crystallographically to the parallel faced hemihedral class of the cubic system. Its common forms are Ai ¢M 4a4

lei Fro. 1. FIG. 2. FIG. 3.

the cube, the octahedron, and the pentagonal dodecahedron. Fig. I d the octahedron {I I I }, and e the pentagonal In fig. 2 -rr {2IO} and lllll areassociatedwith f the dyakis-dodecahedron 1r {321}; whilst Hg. 3 shows a combination of 1r %2lO} and -rr {42I}. The faces of the cube are sometimes striated parallel to the edges between P

and e (fig. I), the striae on each face being therefore at right angles to those of the

adjoining faces, and indicating an oscillatory combination of the cube and pentagonal

dodecahedron. Fig. 4 illustrates a

characteristic twin, formed by two interpenetrating pentagonal dodecahedra. Such

supplementary twins, known in Germany

as “ twins of the Iron Cross, " are commonly brown by superficial conversion into limonite. Pyrites presents a conchoidal fracture, and a very indistinct cubic cleavage. Its hardness is about 6, and its specific gravity 4-9 to 5-2, being rather more than that of marcasite. Moreover, the colour of pyrites is pale brass-yellow, whilst that of marcasite when untarnished may be almost tin-white. From copper-pyrites (chalcopyrite) iron-pyrites is distinguished by its superior hardness and by its paler colour. On ex osure to meteoric influences pyrites commonly becomes brown, by formation of ferric hydrate or limonite, whence the change is called “ limonitization." Such a change is very common on the outcrop of mineral veins, forming what miners call “ gozzan.”' Another kind of alteration which pyrites may suffer has been termed “ vitriolization, " since the products are ferrous sulphate, with free sulphuric acid and sometimes a basic ferric sulphate. It is often said that this saline change is more characteristic of marcasite than of pyrite, but according to H. N. Stokes this statement is incorrect. Contrary, too, to popular belief, he has found a fibrous structure more common in pyrite than in marcasite. In some cases the two forms of iron disulphide occur in intimate association and are difficult to distinguish.

According to the formula FeS2, pyrites contains theoretically 46-67 % of iron and 53-33 of sulphur. Practically, however, it frequently contains other metals, such as copper, cobalt and nickel. Gold is offen present, and in many gold-mining districts the precious metal is obtained mainly from auriferous pyrites. As pyrites, from its brass-yellow colour, is sometimes mistaken for gold, it has been vulgarly called “ fool's gold." Traces of thallium, which are present in some pyrites, may be detected in the fiues of the furnaces where the metal is roasted. Arsenic is an impurity which may be of shows P the cube {100},

dodecahedron 'll' l2IO}.

iq

Ml* if ~iEll“, l[:' 4.

fill: ':“f

ll*, ;2li}|ll'f**l .-. “riff 5

H

V f

FIG. 4.

serious consequence in some of the purposes to which pyrites is applied. The presence of copper, nickel and arsenic is possibly due in many cases to traces o kindred minerals, like chalcopyrite, pentlandite and mispickel.

Pyrites is a mineral of very wide distribution, occurring under varied conditions and probably originating in various ways. It is common in. mineral-Veins, usually associated with quartz, and is often known to miners as “ mundic.” It occurs crystallized, commonly in cubes, in schistose and slaty rocks, and less abundantly in the younger sedimentary deposits. In coal it not infrequently forms bands and nodules known as “brasses, 'f and may also be finely disseminated through the coal as “black pyrites”; but much of the so-called pyrites of coal is really marcasite. Films of pyrites sometimes coat the joint-planes of coal. It is believed that the bluish colour of many clays and limestones is referable to the 'presence of finely divided pyrites, and it is known that certain deposits of blue mud now forming around continental shores owe their colour, in part, to disseminated iron sulphide. Pyritous shales have been largely used in the manufacture of alum, and are therefore known as “ alum-shales.” Many fossils are mineralized with pyrites, which has evidently been reduced by the action of decomposing organic matter on a solution of ferrous sulphate, or perhaps less directly on ferrous carbonate dissolved in water containing carbonic acid, in the presence of certain sulphates. A similar action probably explains the origin of pyrites and marcasite in coal and lignite, in clay and shales, and in limestone like chalk.

Pyrites is largely worked for sake of the sulphur which it contains, and in many cases it has displaced brimstone in the manufacture of sulphuric acid. For this purpose its value depends on the proportion of sulphur present. Pyrites low in sulphur is incapable of sustaining its own combustion without the aid of an external source of heat, and 45% of sulphur is, for economic reasons, usually negarded as the lowest admissible for sulphuric acid manufacture. It is also important for this purpose that the ore should be as free as possible from arsenic (see SULPHURIC ACID).

An extremely important variety of pyrites is that which is more or less cupriferous, and is commonly known commercially as “ copper-pyrites V (q.v.), though distinct mineralogically from that mineral. It consists, indeed, mainly of iron-pyrites, with a notable but variable proportion of copper, sometimes with silver and gold, and not infrequently associated with lead and zinc sulphides. The copper probably exists as disseminated chalcopyrite. Deposits of such cupriferous pyrites are widely distributed and are often of great magnitude. They are generally of lenticular form, and usually occur in or near the contact of eruptive rocks with schists or slates; the presence of the igneous rock being probably connected genetically with their origin. Among the best-known deposits of this character are those in the Huelva district, in the south-west of Spain, including the mines of Rio Tinto, Tharsis, Calanas, &c.; with those of San Domingos in Portugal. At Rio Tinto the ore is divided into three classes:-

(1) The poorest, containing an average of about I%% of copper, which is treated locally by leaching with water and liquor containing ferric sulphate, whereby the copper is dissolved out and afterwards precipitated by pig-iron, whilst the residue is exported as ordinary iron-pyrites. (2) Export ore, with from 2 to 5 % of copper, in which the sulphur, copper and precious metals are utilized, and the residual iron oxide then sold as “ purple ore " for use in iron manufacture. (3) Smelting ore, which averages about 6 % of copper, and is treated metallurgic ally as described under COPPER. The world's annual production of iron-pyrites is about 1,700,000 tons. The largest producer is Spain, with upwards of 3 50,000 tons, including the cupriferous pyrites. France yields about 300,000 tons, largely from the Sain Bel mines, department of the Rhone. Then follows Portugal, with its important output of cupreous pyrites. In the United States the production of pyrites now reaches more than 200,000 tons per annum. The state of Virginia is the chief producer, followed successively by Georgia, North Carolina, Colorado, Massachusetts, Caiifornia, Missouri, New York, &c. From Indiana and