Bismoclite

Bismoclite
Yellow-orange bismoclite on bismuthinite from the Alto do Giz pegmatite, Equador, Rio Grande do Norte, NE-region, Brazil. Approximate size: 20 x 6 x 2 mm.
General
Category Halide mineral
Formula
(repeating unit)		 BiOCl
Strunz classification 3.DC.25
Dana classification 10.2.1.2
Crystal system Tetragonal
Crystal class Ditetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space group P4/nmm
Unit cell a = 3.887 Å,
c = 7.354 Å; Z = 2
Identification
Color Cream-white, greyish, yellowish brown
Crystal habit Platey to thin rectangular crystals, fibrous to columnar, massive
Cleavage {001} perfect
Tenacity Elastic
Mohs scale hardness 2-2.5
Luster Greasy, silky, pearly, dull, earthy
Streak White
Diaphaneity Transparent to translucent
Specific gravity 7.36 (measured), 7.784 (calculated)
Optical properties Uniaxial (-)
Refractive index nω = 2.150 nε = 1.910
Birefringence δ = 0.240
References [1][2][3][4]

Bismoclite is a bismuth oxohalide mineral with formula BiOCl. It is the naturally occurring form of bismuth oxychloride. The name was derived from its chemical constituents. It is a secondary bismuth mineral first thought to be composed of bismuthyl ions (BiO+) and chloride anions, however, the existence of the diatomic bismuthyl ion is doubtful.[5] It is a member of the matlockite group.

It was first described in 1935 from alluvium near bismuth-bearing pegmatites in South Africa.[2] It has been found in association with granite pegmatite and in greisen. Associated minerals include bismutite, mica, jarosite, alunite, cerussite, atacamite, connellite. Occurrences include the type locality at Jackals Water, SW of Prieska, South Africa; Bygoo, Australia; the Tintic district in the East Tintic Mountains of Utah; and from Dalbeattie, Scotland.[3]

Crystal structure

The crystal structure of bismoclite was found to be composed of linked decahedrons, specifically a square antiprism.[6] These decahedrons consist of 2 squares with sides of 3.487 Å (O-O-O-O and Cl-Cl-Cl-Cl) connected by 8 isosceles triangles (O-Cl-O and Cl-O-Cl), with a bismuth atom at the centre.[6] The Bi-O distances and Bi-Cl distances are 2.316 Å and 3.059 Å, respectively. The O-Cl distances in the triangles are 3.249 Å. The decahedrons are linked to each other through shared O-Cl sides.[6]

References

  1. ↑ Mineralienatlas
  2. 1 2 Bismoclite on Mindat.org
  3. 1 2 Bismoclite in the Handbook of Mineralogy
  4. ↑ Bismoclite data on Webmineral
  5. ↑ Nils., Wiberg,; 1859-, Holleman, A. F. (Arnold Frederick), (2001-01-01). Inorganic chemistry. Academic Press. ISBN 0123526515. OCLC 48056955.
  6. 1 2 3 Keramidas, Κ. G.; Voutsas, G. P.; Rentzeperis, P. I. (1993-08-01). "The crystal structure of BiOCl". Zeitschrift für Kristallographie - Crystalline Materials. 205 (1–2). doi:10.1524/zkri.1993.205.12.35. ISSN 2196-7105.


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