Crocoite

Crocoite
Crocoite from Dundas, Tasmania.
General
Category	Chromate mineral
Formula (repeating unit)	Lead Chromate PbCrO4
Strunz classification	07.FA.20
Crystal symmetry	Monoclinic prismatic (2/m)
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
Crystal system	Monoclinic
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.

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.

Crocoite crystal structure

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.

Abundant masses with exceptional examples of crocoite crystals have been found in the Extended Mine at Mount Dundas as well as the Adelaide, Red Lead, West Comet, Platt and a few other Mines at Dundas, Tasmania; they are usually found in long slender prisms, usually about 10–20 mm but rarely up to 100 mm (4 inches) in length, with a brilliant lustre and color. Crocoite is also the official Tasmanian mineral emblem. Other localities which have yielded good crystallized specimens are Congonhas do Campo near Ouro Preto in Brazil, Luzon in the Philippines, Mutare in Mashonaland, near Menzies in Western Australia, plus Brazil, Germany and South Africa.

The relative rarity of crocoite is connected with the specific conditions required for its formation: an oxidation zone of lead ore bed and presence of ultramafic rocks serving as the source of chromium (in chromite). Oxidation of Cr³⁺ into CrO₄²⁻ (from chromite) and decomposition of galena (or other primary lead minerals) are required for crocoite formation. These conditions are relatively unusual.

As crocoite is composed of lead(II) chromate, it is toxic, containing both lead and hexavalent chromium.

Gallery

Examples of crocoite

• 

  Specimen from the Red Lead Mine, Tasmania, Australia

• 

  Crystal intergrowth

• 

  On pyromorphite - Berezovsk - Deposit Topotype

See also

• Bellite

References

•  This article incorporates text from a publication now in the public domain: 
• Bushmakin, A.F., 1996. Crocoite from the Berezovsk gold mines. World of Stones, 10, 28-31
• Bottrill, R.S., Williams, P., Dohnt, S., Sorrell, S. and Kemp, N.R. (2006). Crocoite and associated minerals from Dundas and other locations in Tasmania. Australian Journal of Mineralogy. 12, 59-90

External links

• List of Tasmanian state emblems