Marcasite

The crystals formed by marcasite are generally prismatic and tabular in shape, but cases in which it crystallises in “spearhead” or “cockscomb” twins are by no means rare.

This mineral is opaque, brittle, hard and heavy. Its typically brass-yellow colour tends to darken when exposed to air. Its alteration in air is one of the few characteristics that make it distinguishable from pyrite. It dissolves only with great difficulty in nitric acid.

Marcasite is rather common: it forms within low-temperature hydrothermal veins, in sedimentary rocks by chemical precipitation.

The main uses of this mineral are the production of sulphuric acid, and jewellery and costume jewellery.