Bijvoetite-(Y)

Bijvoetite-(Y)
General
Category	Carbonate mineral
Formula (repeating unit)	(Y,REE)8(UO2)16(CO3)16O8(OH)8•39H2O
Strunz classification	5.EB.20 (10 ed) 5/F.06-30 (8 ed)
Dana classification	16b.2.4.1
Crystal system	Monoclinic
Crystal class	Spheroidal (2) H-M symbol: (2)
Space group	B21
Unit cell	a = 21.23, b = 12.96, c = 44.91 [Å], β = 90.00° (approximated); Z = 4
Identification
Color	Yellow
Crystal habit	Plates
Cleavage	{001}, good
Mohs scale hardness	2
Luster	Vitreous
Streak	Light yellow
Diaphaneity	Transparent to translucent
Density	3.97 (measured)
Optical properties	Biaxal (+)
Refractive index	nα=1.60, nβ=1.65, nγ=1.72 (approximated)
Pleochroism	Colorless (X), pale yellow (Y), deep yellow (Z)
2V angle	84^o (measured)
Other characteristics	Radioactive
References	[1][2][3][4]

Bijvoetite-(Y) is a very rare rare earth and uranium mineral^[2][3] with the formula (Y,REE)₈(UO₂)₁₆(CO₃)₁₆O₈(OH)₈•39H₂O.^[3][4] When compared to the original description, the formula of bijvoetite-(Y) was changed in curse of crystal structure redefinition.^[1] Bijvoetite-(Y) is an example of natural salts containing both uranium and yttrium, the other examples being kamotoite-(Y) and sejkoraite-(Y).^[5][6] Bijvoetite-(Y) comes from Shinkolobwe deposit in Republic of Congo, which is famous for rare uranium minerals. The other interesting rare-earth-bearing uranium mineral, associated with bijvoetite-(Y), is lepersonnite-(Gd).^[2]

Notes on chemistry

Other rare earth elements substituting for yttrium ("REE" in the given formula) are mainly neodymium, samarium, gadolinium, and dysprosium, with minor cerium, europium, terbium and erbium.^[1] This is in slight opposition to the original reported analysis, that had dysprosium, gadolinium and terbium as main substituting REE.^[3]

Occurrence and association

Bijvoetite-(Y) was found in Shinkolobwe dolomite-hosted uranium deposit, Rep. of Congo, where it occurs in an oxidation zone, together with numerous other uranium minerals: lepersonnite-(Gd), becquerelite, curite, kasolite, oursinite, rutherfordine, schoepite, sklodowskite, soddyite, studtite, torbernite, and uranophane.

Crystal structure

Although originally though to be orthorhombic, bijvoetite-(Y) was later shown to be monoclinic. The structural formula of the mineral is [M³⁺₈³⁺(H₂O)₂₅(UO₂)₁₆O₈(OH)₈(CO₃)₁₆](H₂O)₁₄, where M = (Y,REE). The structure has 16 uranium sites, with uranium belonging to near-linear uranyl groups. The important features of the structure are:^[1]

• presence of uranyl pentagonal bipyramids (UPB), formed by coordination of (eight) uranyl groups by three oxide and two hydroxyl anions
• presence of uranyl hexagonal bipyramids (UHB), formed by coordination of another eight uranyl groups by six oxide anions
• presence of uranyl carbonate chain parallel to [100], of a novel type, built of edge-sharing dimers of UPB and UHB, and carbonate groups
• presence of irregular M³⁺Φ_n polyhedra (Φ - unspecified ligand) linking the uranyl carbonate chain, thus forming a compound novel-type sheet parallel to (010)
• location of 14 water molecules in the interlayer space (held by hydrogen bonding)
• bonding of the remaining 25 water molecules to trivalent cations

References

1. Li, Y., Burns, P.C., and Gault, R.A., 2000. A new rare-earth-element uranyl carbonate sheet in the structure of bijvoetite-(Y). The Canadian Mineralogist 38, 153-162.
2. Deliens, M., and Piret, P., 1982. Bijvoetite et lepersonnite, carbonates hydrates d'uranyle et des terres rares de Shinkolobwe, Zaïre. Canadian Mineralogist 20, 231-238
3. "Bijvoetite-(Y) - Handbook of Mineralogy" (PDF). Handbookofmineralogy.org. Retrieved 2016-03-12.
4. "Bijvoetite-(Y): Bijvoetite-(Y) mineral information and data". Mindat.org. Retrieved 2016-03-04.
5. "Kamotoite-(Y): Kamotoite-(Y) mineral information and data". Mindat.org. Retrieved 2016-03-10.
6. "Sejkoraite-(Y): Sejkoraite-(Y) mineral information and data". Mindat.org. Retrieved 2016-03-10.