![cinnabar (HgS), Almaden [130 kb]](0949hs.jpg)
"Rock of the Month # 4, posted October 2001" --- Sample 0949.
Cinnabar, the red, rhombohedral mercury sulphide (HgS), is the principal ore mineral of mercury. It often occurs in association with native mercury and with sulphides such as pyrite. Cinnabar has a cubic polymorph, metacinnabar (metacinnabarite), and the two species commonly occur together. Sample 949 contains more than 80 volume percent cinnabar and lesser metacinnabar, plus appreciable pyrite (FeS2) and stibnite (Sb2S3). Accessory phases include carbonaceous matter and dolomitic carbonate. This sample, roughly 7x5 cm in section, was collected by Chris Graf in 1986 from the Entredicho open-pit mine on the southern limb of the Almaden syncline in Spain, the richest historical Hg mining area in the world.
Almaden, some 225 km southwest of Madrid, is a famous and much-described locality (see, e.g., Crawford, 1988; Saupe, 1990), yet the exact nature, modes of origin and age have been subject of a lively debate which continues to this day. The historical deposits are all located in a middle Paleozoic succession, in the lower Silurian Criadero quartzite. Ordovician black shales may be source rocks for the abundant Hg and S. The region may have seen two episodes of Hg mineralization, an earlier epithermal phase associated with Silurian magmatism and a late epigenetic remobilization associated with the onset of the Hercynian orogeny at the close of the Devonian period.
The widespread use of mercury as an amalgamation agent for the recovery of gold and silver in small-scale alluvial mining has been a serious local environmental problem for some centuries in parts of Latin America and elsewhere (Nriagu, 1993). In addition, the release of Hg both in industrial processes and in areas of naturally Hg-rich bedrock, as in southern China, have focused attention on this intriguing but toxic heavy metal, most familiar in classic thermometers.
These two smaller images are photomicrographs of polished mounts of two mercury ores, taken in reflected light:
![HgS, Almaden [52 kb]](0949ts.jpg)
![HgS, California [61 kb]](1426ts.jpg)
1. Left: Photomicrograph of the Almaden sample. The isotropic metacinnabar is dull grey in this view, and has in any case largely altered to cinnabar along fractures. Note the intense red of the cinnabar along fractures, which also contain pale stibnite, which in more detailed views can also be seen to be undergoing replacement by cinnabar. Magnification 80X, FOV 0.8 mm, PPL-offset XP.
2. Right: Photomicrograph of another rich mercury ore from the Mount Diablo mine, Contra Cosa county, California, USA. Cinnabar, less abundant than its polymorph in this sample, forming from metacinnabar along fractures. The gangue material includes very fine-grained silica, coarser calcite (which displays white internal reflections) and trace pyrite. Magnification 160X, FOV 0.7 mm (taken on a different microscope to Photo 1), PPL-offset XP.
References
CRAWFORD,JW (1988) Famous mineral localities: the Almaden mines, Ciudad Real, Spain. Mineral.Record 19 no.5, 297-302.
NRIAGU,JO (1993) Legacy of mercury pollution. Nature 363, 589.
SAUPE,F (1990) Geology of the Almaden mercury deposit, province of Ciudad Real, Spain. Econ.Geol. 85, 482-510.
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