Realgar

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Arsenic sulphide
Chemical Formula: AsS
Method(s) of Verification: Dolyhir Quarry - XRD (National Museum of Wales, NMW X-1267).

Geological Context:

  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: epithermal polymetallic veins & pipes
Realgar forming crudely crystalline patches lining a thin fracture in Precambrian sedimentary rock from Dolyhir Quarry. Photo D.I. Green, © D.I. Green.
Scanning electron microphotograph of a well-formed realgar crystal. Scale bar 100 microns (0.1 mm). Dolyhir Quarry. Image D.I. Green, © D.I. Green.
Introduction: realgar occurs in low-temperature hydrothermal veins with other sulphide minerals and it is also found in hot spring or fumarolic deposits and as a volcanic sublimate. Associated minerals include calcite, pyrite and other arsenic or antimony compounds such as stibnite, pararealgar and orpiment - the latter two being alteration products. Realgar is a distinctive bright scarlet colour, which one might think would aid in its identification: however the situation is less straightforward than that because upon exposure to daylight realgar immediately begins to alter to similar-looking pararealgar. In time this alteration process and the structural changes it brings about cause the realgar to crumble to powder - incidentally, the name is derived from the Arabic 'rahj al ghar' - 'powder of the mine'. Specimens of realgar therefore require careful storage away from light sources.
Occurrence in Wales: the discovery of realgar in Wales happened relatively recently, in the late 1990s at Dolyhir Quarry in the Welsh Borders. Here, it is but one component of a particularly exotic metalliferous mineral assemblage.

Key Localities:

  • Dolyhir Quarry, Old Radnor, Powys: realgar occurs in thinly-bedded, faulted and folded Precambrian mudstones, in thin (<1 mm) veinlets with occasional associated calcite. Larger part-crystals (to 2 mm) occasionally span slightly wider vuggy veinlets of calcite and quartz and these sometimes overgrow tetrahedral microcrystals of chalcopyrite (National Museum of Wales specimens, collected 2001).

References:

There are no references for this specimen.