The crystals are orthorhombic, with angles similar to those of marcasite; they are often prismatic in habit, and the prism M is usually terminated by the deeply striated faces of an obtuse dome r.
Other species belonging to this isomorphous group of orthorhombic minerals are marcasite (FeS2), lollingite (FeAs 2), safflorite (CoAs2) and rammelsbergite (NiAs 2).
MARCASITE, a mineral with the same chemical composition as pyrites, being iron disulphide FeS2, but crystallizing in the orthorhombic instead of in the cubic system.
Apart from crystalline form, the external characters of marcasite are very similar to those of pyrites, and when distinct crystals are not available the two species cannot always be easily distinguished.
The colour is usually pale bronze-yellow, often rather lighter than that of pyrites; on freshly fractured surfaces of pure marcasite the colour is tin-white, but this rapidly tarnishes on exposure to air.
Arsenical varieties of marcasite, containing up to 5% of arsenic, are known as lonchidite and kyrosite.
Marcasite readily oxidizes on exposure to moist air, with the production of sulphuric acid and a white fibrous efflorescence of ferrous sulphate, and in course of time specimens in collections often became completely disintegrated.
Marcasite is thus the less stable of the two modifications of iron disulphide.
Many experiments have been made with a view to determining the difference in chemical constitution of marcasite and pyrites, but with no very definite results.
It is a noteworthy fact that whilst pyrites has been prepared artificially, marcasite has not.
Marcasite occurs under the same conditions as pyrites, but is much less common.
Whilst pyrites 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.
In the Chalk of the south-east of England nodules of marcasite with a fibrous radiated structure are abundant, and in the Chalk Marl between Dover and Folkestone fine twinned groups of "spear pyrites" are common.
Iron-pyrites was formerly called marcasite, a word variously written marcasin, marchasite, marchesite, marquesite, &c. The two names are now applied to distinct mineral species.
Moreover, the colour of pyrites is pale brass-yellow, whilst that of marcasite when untarnished may be almost tin-white.
It is often said that this saline change is more characteristic of marcasite than of pyrite, but according to H.
A similar action probably explains the origin of pyrites and marcasite in coal and lignite, in clay and shales, and in limestone like chalk.
In marls, cement-stones and argillaceous limestones); (4) sulphide of iron, as pyrite or marcasite (when finely diffused, giving the clay a dark grey-blue colour, which weathers to brown - e.g.
Iron disulphide, FeS2, constitutes the minerals pyrite and marcasite; copper pyrites is (Cu, Fe)S2.
The compound FeS 2 is dimorphous, and the modern practice is to distinguish the cubic forms as pyrites and the orthorhombic as marcasite (q.v.).
For chemical means of distinguishing pyrite from marcasite consult H.
Stokes, "On Pyrite and Marcasite," Bull.