Marcasite Group. Orthorhombic, White Iron Pyrites Composition
Marcasite. White Iron Pyrites
Composition. Iron disulphide. Like pyrite, FeS2 = Sulfur 53.4, iron 46.6.
Crystallization. Orthorhombic. Crystals commonly tabular parallel to basal plane, showing also shor prisms and low brachy domes (Fig. 235). The brahydomes usually striated parallel to the brachy-axis. Often twinned, giving coxcomb and spear shaped group (Fig. 236). Closey related in crystals forms and habit to arsenopiyrite.
Structure. Usually in radiating forms. Often stalactic, having a inner core with radiantin structure and covered on the outside with irregular crystal groups. Also globular, reniform, etc. More rarely in crystals.
Physical Properties. H. = 6-6.5. G. = 4.85-49. Metallic luster. Color pale yellow to almost white, yellow to brown tarnish. Streak grayish black
Tests. Fusible (2.5-3) to a magnetic globule. B. B. on charcoal or in O. T. Gives odor of sulfur dioxide. Much sulfur in C. T. when fine powder is treated by cold nitric acid, and the solution allowed to stand until vigorous action ceases and then boiled, the mineral is decomposed with separation of sulfur. Pyrite treated in the same manner would have been completely dissolved. Recognized usually by its pale yellow color, its crystals or its fibrous structure.
Occurrence. Marcasite is found in metalliferous veins, frequently with lead and zinc ores. Also at times n sedimentary rocks. It is more unstable than pyrite, being easily decomposed, and is not nearly as common in its occurrence. It is deposited at temperatures below 450º C. and from acid solutions (cf. pyrite, p. 164). It is usually formed under surface conditions . it most commonly occurs as replacement deposits in limestone; often in concretions embedded in clays, marls, etc. Found abundantly in clay near Carlsbad and elsewhere in Bohemia; in various places in Saxony; in the chalk marl of Folkestone and Dover, England; with zinc and lead deposits of Joplin, Missouri, and of Mineral Point, Wisconsin; from Galena, Illinois.
Name. Derived from an Arabic word, at one time applied generally to pyrite.
Use. To a slight extent as source of sulfuric acid, etc.
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