Crocoite

Crocoite
Crocoite from Dundas, Tasmania
General
Category	Chromate mineral
Formula (repeating unit)	Lead Chromate PbCrO4
Strunz classification	7.FA.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/n
Unit cell	a = 7.12 Å, b = 7.421 Å, c = 6.8 Å; β = 102.41°; Z = 4
Identification
Color	Orange, red, yellow
Crystal habit	Coarsely crystalline to acicular
Cleavage	Distinct on {110} indistinct on {001} and {100}
Fracture	Conchoidal to uneven
Tenacity	Sectile
Mohs scale hardness	2.5–3
Luster	Adamantine
Streak	Yellowish orange
Diaphaneity	Transparent to translucent
Specific gravity	5.9–6.1
Optical properties	Biaxial (+)
Refractive index	nα = 2.290(2) nβ = 2.360(2) nγ = 2.660(2)
Birefringence	δ = 0.370
Pleochroism	Weak
References

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.

Crocoite is commonly found as large, well-developed prismatic adamatine 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.