Zaïrite
Zaïrite from Eta-Etu, Kivu, Democratic Republic of Congo (Zaïre), (field of view 6 mm)
General
CategoryPhosphate minerals
Formula
(repeating unit)
Bi(Fe3+,Al)3[(OH)6|(PO4)2]
IMA symbolZaï[1]
Strunz classification8.BL.13
Crystal systemTrigonal
Crystal classHexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space groupR3m
Unit cella = 7.015, c = 16.365 [Å]; Z = 3
Identification
Formula mass646.86 g/mol
ColorGreenish, greenish white, yellow green.
CleavageNone observed
FractureIrregular, uneven
TenacityBrittle
Mohs scale hardness4.5
LusterVitreous, resinous
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity4.37
Optical propertiesUniaxial(-)
Refractive indexnω = 1.820 - 1.830 nε = 1.810
Birefringenceδ = 0.0100-0.0200
Other characteristicsOccurrence: Rare mineral in the weathering zone of quartz wolframite deposits
References[2][3][4]

Zaïrite is a phosphate mineral with the chemical formula Bi(Fe3+,Al)3[(OH)6|(PO4)2].[2] The name was given from where it was locally discovered in Eta-Etu, Kivu, Congo (Zaïre) in 1975.[3]

Properties

Zaïrite crystallizes in the trigonal crystal system, which means it contains three equal horizontal axes with angles of 120° between them.[5] The mineral has a trigonal-hexagonal scalenohedron shape.[6] Zairite belongs to the uniaxial optical class. It means the light and vibrations passing through the mineral only has one direction it follows and it travels at the same speed.[5]

Occurrence

The mineral was first described from the Eta-Etu district, northern Kivu Province, Democratic Republic of the Congo (then Zaire) usually in the weathering areas of quartz wolframite deposits where it occurs with native bismuth, bismutite, quartz and mica.[4] Zaïrite is found in granite pegmatites, which are igneous coarse-grained rocks. It has also been reported from Kreuzberg Mountain in Upper Palatinate, Bavaria, Germany.[2]

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. 1 2 3 Mindat.org
  3. 1 2 Webmineral data
  4. 1 2 Handbook of Mineralogy
  5. 1 2 Klein, C., Dutrow, B. (2007) The 23rd edition of the Manual of Mineral Science, 131 p. John Wiley and Sons, Inc. Hoboken, New Jersey, U.S.A.
  6. McKie, D., McKie, C. (1986) Essentials of Crystallography. III. Blackwell Scientific Publishers, p.69
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