Guilleminite

Guilleminite (Ba(UO2)3(SeO3)2(OH)4•3H2O) is a uranium mineral named by R. Pierrot, J. Toussaint, and T. Verbeek in 1965 in honor of Jean Claude Guillemin (1923–1994), a chemist and mineralogist. It is a rare uranium/selenium mineral found at the Musonoi Mine in the Katanga Province of the Democratic Republic of the Congo.[3]

Guilleminite
A cuprosklodowskite vug filled with well formed dark green vandenbrandeite crystals. These are dusted over with small yellow crystals of guilleminite.
General
CategoryOxide minerals
Formula
(repeating unit)		Ba(UO2)3(SeO3)2(OH)4•3H2O
IMA symbolGul[1]
Strunz classification4.JJ.10
Dana classification34.07.03.01
Crystal systemOrthorhombic
Crystal classPyramidal (mm2)
(same H-M symbol)
Space groupP21nm
Unit cell869.48 ų
Identification
ColorBright yellow, greenish yellow, yellow
CleavagePerfect on {100}, good on {010}
FractureBrittle
TenacityBrittle
Mohs scale hardness2
LusterWaxy, greasy, dull, earthy
DiaphaneityTransculent
Specific gravity4.88
Optical propertiesBiaxial (-)
Refractive indexnα = 1.720 nβ = 1.798 nγ = 1.805
Birefringence0.085
PleochroismStrong
2V angleMeasured 35°, calculated 32°
Dispersionr > v strong
Other characteristics Radioactive
References[2][3][4]

This secondary mineral also includes barium in its structure, in addition to selenium and uranium. It is bright yellow in colour and usually has an acicular crystal habit. It has a Mohs hardness of 2–3.[5]

Pleochroism

Guilleminite shows strong pleochroic attributes. Depending on the axis the gem is seen, guilleminite on the X axis can be seen in a bright yellow color, on the Y axis can be seen yellow, and on the Z axis is seen as a colorless gem.

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. Guilleminite: Guilleminite mineral information and data on Mindat.org
  4. Webmineral data
  5. Pierrot R, Toussaint J, Verbeek T. Bull. Soc. Franc. Mineral. Crist. 1965; 88: 132.
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