Crocoite from Dundas, Tasmania.
Crystal intergrowth of the secondary lead mineral crocoite
Crocoite is a mineral consisting of lead chromate, PbCrO4, and crystallizing in the monoclinic system. It is sometimes used as a paint, being identical in composition with the artificial product chrome yellow; it is the only chromate of any importance found in nature. It was discovered at Berezovsk near Ekaterinburg in the Urals in 1766; and named crocoise by F. S. Beudant in 1832, from the Greek ??????, saffron, in allusion to its color, a name first altered to crocoisite and afterwards to crocoite. It is found as well-developed crystals, but there are most often poorly terminated, and are of a bright hyacinth-red color, which are translucent and have an adamantine to vitreous lustre. On exposure to light much of the translucency and brilliancy is lost. The streak is orange-yellow; Mohs hardness is 2.5-3; and the specific gravity is 6.0. In the Urals the crystals are found in quartz-veins traversing granite or gneiss. Other localities which have yielded good crystallized specimens are Congonhas do Campo near Ouro Preto in Brazil, Luzon in the Philippines, and Umtali in Mashonaland.
Gold is often found associated with this mineral. Crystals far surpassing in beauty any previously known have been found in the Adelaide Mine at Dundas, Tasmania; they are long slender prisms, 3 or 4 in. in length, with a brilliant lustre and color. Crocoite is also the official Tasmanian mineral emblem.
Associated with crocoite at Berezovsk are the closely allied minerals phoenicochroite and vauquelinite. The former is a basic lead chromate, Pb2CrO5, and the latter a lead and copper phosphate-chromate, Pb2CuCrO4PO4OH. Vauquelinite forms brown or green monoclinic crystals, and was named after L. N. Vauquelin, who in 1797 discovered (simultaneously with and independently of M. H. Klaproth) the element chromium in crocoite.
Relative rarity of crocoite is connected with specific conditions required for its formation: an oxidation zone of lead ore bed and presence of ultramafic rocks serving as the source of Cr (in chromite). Oxidation of Cr3+ into CrO42- (from chromite) and decomposition of galena (or other primary Pb minerals) are required for crocoite formation.
See also
References
External links
| |
