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!
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.
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