Demesmaekerite is a rare uranium selenite mineral with the chemical formula: Pb₂Cu₅(UO₂)₂(SeO₃)₆(OH)₆·2H₂O.

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Demesmaekerite
A specimen of demesmaekerite associated with blue chalcomenite crystals
General
Category	Oxide mineral
Formula (repeating unit)	Pb2Cu5(UO2)2(SeO3)6(OH)6·2H2O
IMA symbol	Dmm[1]
Strunz classification	4.JJ.20
Dana classification	34.07.06.01
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 11.94 Å, b = 10.02 Å, c = 5.62 Å; α = 90°, β = 100°, γ = 91.91°
Identification
Formula mass	2,172.01 gm
Color	Green to olive green – turns brownish on dehydration
Crystal habit	Elongated, striated, typically in radial aggregates
Cleavage	None
Mohs scale hardness	3–4
Diaphaneity	Translucent, opaque
Specific gravity	5.28
Density	Measured 5.28(4), calculated 5.45
Optical properties	Biaxial (+)
Refractive index	nα = 1.835 nγ = 1.910
Pleochroism	Visible
Dispersion	Strong
Other characteristics	Radioactive
References	[2][3]

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It is named after the Belgian mineralogist Gaston Demesmaeker, who worked at the Musonoi Mine in Katanga. It is a secondary mineral which contains lead, copper and selenium, it is a bottle green to brown/yellow color, its crystal habit varies depending on where it is found. It has pleochroic attributes, which means depending on which axis it is seen, the gem displays different colors, which is an optical phenomenon. On the X axis it displays a yellow-green color, and on the Y the gem is seen in a brown color.^[2] Demesmaekerite has a very strong radioactivity, 1,629,108.74, measured in GRapi (Gamma Ray American Petroleum Institute Units). It is mostly made out of oxygen (22.1%), uranium (21.92%) which causes its irradiative attributes, selenium (21.81%), lead (19.08%) which is a poisonous chemical element and copper (14.63%), but also contains hydrogen (0.46%).^[4]

It can be found associated with other rare selenium-bearing uranium ores, such as haynesite, guilleminite, marthozite and piretite.^[3]