Francevillite

Francevillite is a uranyl-group vanadate mineral in the tyuyamunite series. Its chemical formula is (Ba,Pb)(UO
2
)
2
V
2
O
8
·5(H
2
O)
. Francevillite is a strongly radioactive mineral. It is typically orange, yellow or brownish yellow. It forms a series with curienite.[1]

Francevillite
Yellow crystals of francevillite on matrix with small mounanaite crystals (specimen size, 4 x 3.5 x 1 cm)
General
CategoryVanadate mineral
Formula
(repeating unit)
(Ba,Pb)(UO
2
)
2
V
2
O
8
·5(H
2
O)
Strunz classification4.HB.15
Dana classification40.02a.27.01
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPcan
Unit cella = 10.41, b = 8.51
c = 16.76 [Å]; Z = 4
Identification
ColorLemon-yellow, yellow-orange, orange, greenish yellow; green, brown
Crystal habitAggregates and incrustations of crystals, also massive, in veinlets and as impregnations
CleavageOn {001}, perfect
Mohs scale hardness3
LusterAdamantine - pearly
StreakLight yellow
DiaphaneitySemitransparent
Specific gravity4.55
Optical propertiesBiaxial (-)
Refractive indexnα = 1.750 - 1.785 nβ = 1.910 - 1.952 nγ = 1.945 - 2.002
Birefringenceδ = 0.195 - 0.217
PleochroismX = colorless; Y = Z = yellow
2V angleMeasured: 52°, Calculated: 46° to 52°
Other characteristics Radioactive
References[1][2][3][4]

Occurrence

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), chervetite (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

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