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.
Other species belonging to this isomorphous group of orthorhombic minerals are marcasite (FeS2), lollingite (FeAs 2), safflorite (CoAs2) and rammelsbergite (NiAs 2).
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.
For chemical means of distinguishing pyrite from marcasite consult H.
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.
A similar action probably explains the origin of pyrites and marcasite in coal and lignite, in clay and shales, and in limestone like chalk.
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.
It is a noteworthy fact that whilst pyrites has been prepared artificially, marcasite has not.
It is often said that this saline change is more characteristic of marcasite than of pyrite, but according to H.
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.