Crocoite

Crocoite
Crocoite from Dundas, Tasmania.
General
Category	Chromate mineral
Chemical formula	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	[1][2][3]

Crocoite is a mineral consisting of lead chromate, PbCrO₄, and crystallizing in the monoclinic crystal system. It is sometimes used as a paint, being identical in composition with the artificial product chrome yellow. It was discovered at Berezovsky deposit near Ekaterinburg in the Urals in 1766; and named crocoise by F. S. Beudant in 1832, from the Greek κροκος, saffron, in allusion to its color, a name first altered to crocoisite and afterwards to crocoite. It is found as well-developed crystals, although these are usually poorly terminated. Crystals are of a bright hyacinth-red color, 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 Mutare in Mashonaland.

Crocoite specimen from the Red Lead Mine, Tasmania, Australia

Gold is often found associated with this mineral. Exceptional examples of crocoite crystals have been found in the Adelaide Mine at Dundas, Tasmania; they are long slender prisms, 3 to 4 inches (7.5 to 10 cm) in length, with a brilliant lustre and color. Crocoite is also the official Tasmanian mineral emblem.

Associated with crocoite at Berezovsk are the similar 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.

Crystal intergrowth of the secondary lead mineral 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 chromium (in chromite). Oxidation of Cr³⁺ into CrO₄^2- (from chromite) and decomposition of galena (or other primary lead minerals) are required for crocoite formation.

Crocoite on pyromorphite - Berezovsk - Deposit Topotype

See also

• Bellite

References

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. 

1. ^ http://rruff.geo.arizona.edu/doclib/hom/crocoite.pdf Handbook of Mineralogy
2. ^ http://www.mindat.org/min-1157.html Mindat
3. ^ http://webmineral.com/data/Crocoite.shtml Webmineral data

External links

• List of Tasmanian state emblems