Quartz, Pyrite and Pyrrhotite

Quartz (see previous entries)

Pyrite

Pyrite, a very well-known and widespread mineral, forms cubic crystals, with smooth or striated faces, octahedral, pentagonal dodecahedral. It sometimes also occurs in compact masses, granular, stalactitic and nodular. This mineral is hard, brittle, heavy and opaque.

If struck with a metal object it produces sparks. It melts easily, releasing a strong smell of zinc and, if placed in humid environments it turns into sulphuric acid and other products.

The fact that pyrite is so frequent is due to its forming in various geological environments: in hydrothermal veins, in metamorphic rocks, magmatic and sedimentary rocks.

Pyrite is used for the extraction of gold and nickel and for the production of sulphuric acid, copper, iron and steel.

Pyrrhotite

Pyrrhotite forms flattened tabular crystals but is often found in massive and granular aggregates, Its colour, which varies from bronze yellow to copper red, alters in air and becomes brown. It is brittle, heavy, with a metallic lustre, magnetic (hence the name magnetopyrite). It melts easily into a strongly magnetic black mass.

It generally forms within eruptive igneous rocks, basic and ultrabasic, associated with pyrite, galena and other sulphides, more rarely it can form within hydrothermal veins.

Pure pyrrhotites are of rather little interest, but nickel-bearing ones, are used for the extraction of nickel and only rarely of cobalt and platinum.