REALGAR & PARAREALGAR
One of the prettiest rocks I’ve ever seen is this specimen from the Getchell Mine of northern Nevada, USA. The rock itself (gray portion) is marble. The large whitish gray crystals at right and right-center and top are calcite (CaCO3 - calcium carbonate). The red crystals are realgar (AsS; aka As4S4 - arsenic sulfide). The yellowish and orangish areas are pararealgar, which has the same chemical formula as realgar. Realgar is unstable when exposed to light, so all the orangish-yellowish pararealgar you see in the rock used to be red realgar.
Geology - Cambrian-aged Preble Formation limestone, contact metamorphosed into marble by the Cretaceous-aged Osgood Granodiorite Stock. During the late Middle Eocene, at 39 million years ago, the rock was subjected to Carlin-type mineralization by fluids moving along the Getchell Fault (a major normal fault formed during Basin & Range extensional tectonics). The mineralization event precipitated the calcite and the realgar (plus a teeny-tiny amount of disseminated gold, but not enough to make this sample a gold ore).
Locality: 4950-194 stope of the Getchell Mine, northern end of the Osgood Mountains, eastern Humboldt County, northern Nevada, USA.
Realgar (red) and pararealgar (orange & yellow) and calcite crystals on marble (14.2 cm across).
Realgar (red) and calcite (grayish-whitish) (field of view ~3.5 cm across)
Collected & generously donated by Keith Wood.
Most info. from Keith Wood.