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Crystal structure of the (REE)-uranyl carbonate mineral kamotoite-(Y)

Published online by Cambridge University Press:  02 January 2018

Jakub Plášil  and
Václav Petříček
Jakub Plášil*
Affiliation:
Institute of Physics ASCR, v.v.i., Na Slovance 2, CZ-182 21 Prague 8, Czech Republic
Václav Petříček
Affiliation:
Institute of Physics ASCR, v.v.i., Na Slovance 2, CZ-182 21 Prague 8, Czech Republic
*
*E-mail: plasil@fzu.cz
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Abstract

Kamotoite-(Y) is a rare supergene product of uraninite hydration–oxidation weathering and its structure is unknown. Based on single-crystal X-ray diffraction data collected with high-redundancy using a microfocus source, kamotoite-(Y) is monoclinic, has space group P21/n,with a = 12.3525(5), b = 12.9432(5), c = 19.4409(7) Å, β = 99.857(3)°, V = 3069.8(2) Å3 and Z = 4. Crystals are pervasively twinned (two-fold rotation around [0.75 0 0.75]), giving a strongly pseudo-orthorhombic diffractionpattern. The pseudoorthorhombic pattern can be described with an orthorhombic super-cell (transformation matrix 0,1,0/1,0,1/3,0,1), approximately four times larger in volume then a true monoclinic unit cell. This unit-cell is the same as the cell given elsewhere for the structure of bijvoetite-(Y),another (REE)-containing uranyl carbonate. The successful structure solution and refinement (R = 0.044 for 6294 unique observed reflections), carried out using our choice of unit cell, as well as the superstructure refinement and comparison of the original structure data forbijvoetite-(Y) reveal that these two crystal structures are identical. The crystal structure of kamotoite-(Y) consists of electroneutral sheets of the bijvoetite-(Y) uranylanion topology and an interlayer with H2O molecules not-coordinated directly to any metal cation. Despite determinationof the kamotoite-(Y) structure and demonstration that bijvoetite-(Y) has the same structure, the identity of these two minerals cannot be proved without additional study of the holotype material.

Keywords

kamotoite-(Y)uranyl carbonaterare-earth elementscrystal structure
Type
Research Article
Information
Mineralogical Magazine , Volume 81 , Issue 3 , June 2017 , pp. 653 - 660
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2017

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