Guilleminite
A cuprosklodowskite vug filled with well formed dark green vandenbrandeite crystals. These are dusted over with small yellow crystals of guilleminite.
General
Category	Oxide minerals
Formula (repeating unit)	Ba(UO2)3(SeO3)2(OH)4•3H2O
IMA symbol	Gul[1]
Strunz classification	4.JJ.10
Dana classification	34.07.03.01
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) (same H-M symbol)
Space group	P21nm
Unit cell	869.48 ų
Identification
Color	Bright yellow, greenish yellow, yellow
Cleavage	Perfect on {100}, good on {010}
Fracture	Brittle
Tenacity	Brittle
Mohs scale hardness	2
Luster	Waxy, greasy, dull, earthy
Diaphaneity	Transculent
Specific gravity	4.88
Optical properties	Biaxial (-)
Refractive index	nα = 1.720 nβ = 1.798 nγ = 1.805
Birefringence	0.085
Pleochroism	Strong
2V angle	Measured 35°, calculated 32°
Dispersion	r > v strong
Other characteristics	Radioactive
References	[2][3][4]

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]

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