Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Arsenic sulphide
Chemical Formula: AsS
Method(s) of Verification: Dolyhir Quarry - XRD (National Museum of Wales, NMW X-1267).
- 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.
- 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).
There are no references for this specimen.