Crocoite

Crocoite is a mineral consisting of lead chromate, PbCrO₄, and crystallizing in the monoclinic crystal system. It is identical in composition with the artificial product chrome yellow used as a paint pigment.

Description

Crocoite is commonly found as large, well-developed prismatic crystals, although in many cases are poorly terminated. Crystals are of a bright hyacinth-red color, translucent, and have an adamantine to vitreous lustre. On exposure to UV light some 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.

It was discovered at the Berezovskoe Au Deposit (Berezovsk Mines) near Ekaterinburg in the Urals in 1766; and named crocoise by F. S. Beudant in 1832, from the Greek κρόκος (krokos), saffron, in allusion to its color, a name first altered to crocoisite and afterwards to crocoite. In the type locality the crystals are found in gold-bearing quartz-veins traversing granite or gneiss and associated with crocoite are quartz, embreyite, phoenicochroite and vauquelinite. Phoenicochroite is a basic lead chromate, Pb₂CrO₅ with dark red crystals, and vauquelinite a lead and copper phosphate-chromate, Pb₂CuCrO₄PO₄OH, with brown or green monoclinic crystals. Vauquelinite was named after L. N. Vauquelin, who in 1797 discovered (simultaneously with and independently of M. H. Klaproth) the element chromium in crocoite.