Guilleminite

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	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 Å3
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]

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.