Guilleminite (Ba(UO₂)₃(SeO₃)₂(OH)₄•3H₂O) is a

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

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 Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, ...

mineral found at the Musonoi Mine in the

Katanga Province Katanga was one of the four large provinces created in the Belgian Congo in 1914. It was one of the eleven provinces of the Democratic Republic of the Congo between 1966 and 2015, when it was split into the Tanganyika Province, Tanganyika, Hau ...

of the

Democratic Republic of the Congo The Democratic Republic of the Congo (french: République démocratique du Congo (RDC), colloquially "La RDC" ), informally Congo-Kinshasa, DR Congo, the DRC, the DROC, or the Congo, and formerly and also colloquially Zaire, is a country in ...

.Guilleminite: Guilleminite mineral information and data on Mindat.org
/ref> This secondary mineral also includes

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

in its structure, in addition to selenium and uranium. It is bright yellow in colour and usually has an acicular

crystal habit In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crystal group. The habit of a crystal is dependent on its crystallographic form and growth conditions, which generally creates irregularities due to l ...

. It has a

Mohs hardness The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...

of 2–3.Pierrot R, Toussaint J, Verbeek T. ''Bull. Soc. Franc. Mineral. Crist.'' 1965; 88: 132.
/ref>

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

{{reflist Uranium(VI) minerals Selenium minerals Barium minerals Orthorhombic minerals Minerals in space group 31 Minerals described in 1965