Phosgenite

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Phosgenite
Phosgenite-34631.jpg
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[edit]

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[edit]

  1. ^ a b 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