Bismoclite

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.[6] It is a member of the matlockite group.

Bismoclite
Yellow-orange bismoclite interspersed with bismuthinite from the Alto do Giz pegmatite, Equador, Rio Grande do Norte, NE-region, Brazil. Approximate image width: 5 mm.
General
CategoryHalide mineral
Formula
(repeating unit)		BiOCl
IMA symbolBmc[1]
Strunz classification3.DC.25
Dana classification10.2.1.2
Crystal systemTetragonal
Crystal classDitetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space groupP4/nmm
Unit cella = 3.887 Å,
c = 7.354 Å; Z = 2
Identification
ColorCream-white, greyish, yellowish brown
Crystal habitPlatey to thin rectangular crystals, fibrous to columnar, massive
Cleavage{001} perfect
TenacityElastic
Mohs scale hardness2-2.5
LusterGreasy, silky, pearly, dull, earthy
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity7.36 (measured), 7.784 (calculated)
Optical propertiesUniaxial (-)
Refractive indexnω = 2.150 nε = 1.910
Birefringenceδ = 0.240
References[2][3][4][5]

It was first described in 1935 from alluvium near bismuth-bearing pegmatites in South Africa.[3] 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.[4]

Crystal structure

The crystal structure of bismoclite was found to be composed of linked decahedrons, specifically a square antiprism.[7] 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.[7] 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.[7]

References
  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Mineralienatlas
  3. Bismoclite on Mindat.org
  4. Bismoclite in the Handbook of Mineralogy
  5. Bismoclite data on Webmineral
  6. Wiberg, Nils; Holleman, A. F. (2001-01-01). Inorganic chemistry. Academic Press. ISBN 0123526515. OCLC 48056955.
  7. 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): 35. Bibcode:1993ZK....205...35K. doi:10.1524/zkri.1993.205.12.35. ISSN 2196-7105. S2CID 96526873.


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