Phosgenite

Phosgenite
Crystal of phosgenite from the Monteponi Mine, Iglesias, Sardinia, Italy (size: 3.0 x 3.0 x 2.5 cm)
General
Category	Carbonate minerals
Formula (repeating unit)	(PbCl)2CO3
Strunz classification	05.BE.20
Crystal symmetry	Tetragonal (4/m 2/m 2/m) ditetragonal dipyramidal
Unit cell	a = 8.16Å, c = 8.883(6) Å; Z = 4
Identification
Color	Pale yellow to yellowish brown, pale brown, smoky brown, smoky violet; colorless, pale rose, gray, yellowish gray, pale green
Crystal habit	Short prismatic crystals, granular, massive
Crystal system	Tetragonal
Cleavage	Distinct on {001} and {110}, indistinct on {100}
Fracture	Conchoidal
Tenacity	Sectile, flexible perpendicular to {001}
Mohs scale hardness	2 – 3
Luster	Adamantine
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	6.12 – 6.15
Optical properties	Uniaxial (+); anomalously biaxial if strained
Refractive index	nω = 2.118 nε = 2.145
Birefringence	δ = 0.027
Pleochroism	Weakly pleochroic with O - reddish and E - greenish in thick sections.
Ultraviolet fluorescence	Fluoresces yellow under LW and SW UV
Solubility	Soluble in dilute nitric acid with effervescence, decomposes slowly in cold water
References	[1][2][3]

Phosgenite is a rare mineral consisting of lead chlorocarbonate, (PbCl)₂CO₃. The tetragonal crystals are prismatic or tabular in habit: they are usually colorless and transparent, and have a brilliant adamantine lustre. Sometimes the crystals have a curious helical twist about the tetrad or principal axis. The hardness is 3 and the specific gravity 6.3. The mineral is rather sectile, and consequently was earlier known as corneous lead.

Name and occurrence

An example of the mineral Phosgenite on display at the Royal Ontario Museum

The name phosgenite was given by August Breithaupt in 1820, from phosgene, carbon oxychloride, because the mineral contains the elements carbon, oxygen and chlorine.

It was found associated with anglesite and matlockite in cavities within altered galena in a lead mine at Cromford, near Matlock: hence its common name cromfordite.^[4] Crystals are also found in galena at Monteponi near Iglesias in Sardinia, and near Dundas in Tasmania. It has also been reported from Laurium, Greece; Tarnowitz, Poland; the Altai district, Siberia; the Touissit mine, near Oujda, Morocco; Sidi Amor ben Salem, Tunisia; Tsumeb, Namibia; Broken Hill, New South Wales; and Boleo, near Santa Rosalia, Baja California. In the US it has been reported from the Terrible mine, Custer County, Colorado; the Stevenson-Bennett mine, Organ Mountains, Doña Ana County, New Mexico; and the Mammoth mine, Tiger, Pinal County, Arizona.^[1]

Crystals of phosgenite, and also of the corresponding bromine compound PbBr₂CO₃, have been prepared artificially.

References

1. ↑ 1.0 1.1 Handbook of Mineralogy
2. ↑ Mindat.org
3. ↑ Webmineral data
4. ↑ "Phosgenite and Matlockite in Derbyshire (Part 1). T. Bridges, M.E. Smith. Journal of the Russell Society Volume 1, No. 2, p7-14, 1983 Retrieved on 2011-01-11

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