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Revision of the crystal structure of ulrichite, CaCu²⁺(UO₂)(PO₄)₂·4H₂O

Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstr. 14, A-1090 Wien, Austria

^* E-mail: uwe.kolitsch{at}univie.ac.at

The crystal structure of ulrichite, CaCu²⁺(UO₂)(PO₄)₂·4H₂O (space group P2₁/c, a = 12.784(3), b = 6.996(1), c = 13.007(3) Å , ß = 91.92(1)°, V = 1162.7(4) ų, Z = 4) was redetermined using X-ray diffraction data measured from a twinned crystal with Mo-K radiation and a CCD area detector (2510 unique reflections with F_o > 4(F_o), R1 = 8.8%). Ulrichite crystallizes in space group P2₁/c rather than C2/m reported previously. The newly determined atomic positions give reasonable coordination polyhedra. One unique Ca atom is irregularly coordinated by eight O atoms (<Ca–O> = 2.46 Å). One unique U atom shows a [2+5] coordination with characteristic bond angles and lengths (1.806(11) Å, 1.842(12) Å and five bonds between 2.252(15) and 2.441(11) Å). Furthermore, the structure contains groups in which strongly elongated CuO₆ ‘octahedra’ (also describable as CuO₄ squares) are corner-linked to two PO₄ tetrahedra via two opposite, equatorial O atoms. Edge- and corner-sharing UO₇, CaO₈ and PO₄ polyhedra form heteropolyhedral sheets parallel to (001) that are linked to adjacent sheets via the CuO₆ ‘octahedra’ and hydrogen bonds.

KEYWORDS: ulrichite, crystal structure, twinning, uranium, phosphates

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