MARCASITE, a mineral with the same chemical composi tion as pyrite, being iron disulphide but crystallizing in the orthorhombic instead of in the cubic system. The name is of Arabic origin and was long applied to crystallized pyrites in gen eral. It was known to G. Agricola in 1546 as Wasserktes or Wetss erkies and Leberkies, and it has been variously known as white pyrites, hepatic pyrites, lamellar pyrites, radiated pyrites (Ger man Strahikies) and prismatic pyrites.
The crystals are isomorphous with mispickel (q.v.), but only rarely are they distinctly developed and simple. Often they are twinned on a prism plane producing pentagonal stellate groups of five crystals. This frequent twinning gives rise to characteristic forms, with many re-entrant angles, to which the names "spear pyrites" and "cockscomb pyrites" are applied. The commonest state of aggregation is that of radially arranged fibres, the external surface of the mass being globular, nodular or stalactitic in form.
Apart from crystalline form, the external characters of mar casite are very similar to those of pyrite, and when distinct crys tals are not available the two species cannot always be easily distinguished. The colour is usually pale bronze-yellow, often rather lighter than that of pyrite ; on freshly fractured surfaces of pure marcasite the colour is tin-white, but this rapidly tarnishes on exposure to air. The lustre is metallic and brilliant, the streak greyish or brownish-black. The hardness (6-61) is the same as that of pyrite, and the specific gravity (4.8-4.9) as a rule rather less. It readily oxidizes on exposure to moist air, with the produc
tion of sulphuric acid and a white fibrous efflorescence of ferrous sulphate, and in course of time specimens in collections often become completely disintegrated. In nature it is frequently altered to limonite. Marcasite is thus the less stable of the two modifica tions of iron disulphide. Many experiments have been made with a view to determining the difference in chemical constitution of marcasite and pyrite, but with no very definite results. Marcasite has been prepared artificially from acid solutions, whilst pyrite is formed only from slightly acid or neutral solutions.
Marcasite occurs under the same conditions as pyrite, but is much less common. While pyrite is found abundantly in the older crystalline rocks and slates, marcasite is more abundant in clays, and has often been formed as a concretion around organic remains. It is abundant, for example, in the plastic clay of the brown coal formation at Littmitz, near Carlsbad, where it has been extensively mined for the manufacture of sulphur and ferrous sulphate. In the chalk of the south-east of England nodules of marcasite with a fibrous radiated structure are abundant (these bodies being often mistaken for "thunderbolts" or meteorites), and in the chalk marl between Dover and Folkestone fine twinned groups of "spear pyrites" are common. The mineral is also met with in metalliferous veins; for example the "cockscomb pyrites" of the lead mines of Derbyshire and Cumberland.