Francevillite

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Francevillite
Francevillite-Mounanaite-Curienite-20877.jpg
Yellow crystals of francevillite on matrix with small mounanaite crystals (specimen size, 4 x 3.5 x 1 cm)
General
Category Vanadate mineral
Formula
(repeating unit)
(Ba,Pb)(UO2)2V2O8·5(H2O)
Strunz classification 4.HB.15
Dana classification 40.02a.27.01
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space group Pcan
Unit cell a = 10.41, b = 8.51
c = 16.76 [Å]; Z = 4
Identification
Color Lemon-yellow, yellow-orange, orange, greenish yellow; green, brown
Crystal habit Aggregates and incrustations of crystals, also massive, in veinlets and as impregnations
Cleavage On {001}, perfect
Mohs scale hardness 3
Luster Adamantine - pearly
Streak Light yellow
Diaphaneity Semitransparent
Specific gravity 4.55
Optical properties Biaxial (-)
Refractive index nα = 1.750 - 1.785 nβ = 1.910 - 1.952 nγ = 1.945 - 2.002
Birefringence δ = 0.195 - 0.217
Pleochroism X = colorless; Y = Z = yellow
2V angle Measured: 52°, Calculated: 46° to 52°
Other characteristics Radioactive
References [1][2][3][4]

Francevillite is a uranyl-group vanadate mineral in the tyuyamunite series. Its chemical formula is (Ba,Pb)(UO2)2V2O8·5(H2O). Francevillite is a strongly radioactive mineral. It is typically orange, yellow or brownish yellow. It forms a series with curienite.[1]

Occurrence[edit]

Francevillite occurs in the oxidized zone of a lead-bearing uraniumvanadium deposits.[1] Francevillite was first described in 1957 for an occurrence in its type locality of the idle Mounana uranium mine, near Franceville, Haut-Ogooué, Gabon and was named for the city.[2][3]

At its type locality it is associated with curienite (a closely related uranyl vanadate), chevetite (a lead vanadate), and mounanaite (another lead vanadate). At other localities, francevillite is associated with duttonite, vanuralite, mottramite, carnotite, dewindtite, torbernite, uranopilite, johannite and kasolite.[1]

References[edit]