J.S. Mason - Central Wales Orefield: Early pyritohedral pyrite; late zoned pyrite

Pyrite is widespread in the Central Wales veins. In the early, complex polymetallic veins, it typically forms pyritohedra (L), up to 1 or 2 millimetres in places. It is one of the earliest sulphides to precipitate in these cases, but occurs only in very minor amounts.

The main iron sulphide mineralisation in Central Wales was the final hypogene mineralising event, depositing abundant marcasite and pyrite. They are so abundant in some areas, such as at Ystumtuen and Cwmrheidol, to cause serious environmental problems due to acid mine drainage. See the
Conservation page for a case study. Central Wales marcasite is so unstable that polished sections often slowly explode. All in all a horrible mineral, especially for museum curators!

Thankfully this sample (R), from Eaglebrook mine, had very little marcasite in it. It reveals the beautiful zonation of 0.25mm pyrite crystals. The brown bands are probably nickel-enhanced and resemble bravoite from other localities. Electron microprobe analysis is required in order to confirm this assertion.