1. The crystal structure of synthetic autunite, Ca[[(U[O.sub.2])(P[O.sub.4]).sub.2]][([H.sub.2]O).sub.11]
- Author
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Locock, Andrew J. and Burns, Peter C.
- Subjects
Mineralogy -- Research ,Crystals ,Earth sciences - Abstract
Autunite, Ca[[(U[O.sub.2])(P[O.sub.4]).sub.2]][([H.sub.2]O).sub.11], is amongst the most abundant and widely distributed of the uranyl phosphate minerals, yet because of its pseudo-tetragonal symmetry and rapid dehydration in air, the details of its symmetry, stoichiometry, and structure were previously uncertain. The crystal structure of synthetic autunite was solved by direct methods and refined by full-matrix least-squares techniques to agreement indices [R.sub.1] = 0.041, calculated for the 1497 unique observed reflections (|[F.sub.o]| [greater than or equal to] 4[[sigma].sub.F]), and w[R.sub.2] = 0.119 for all data. Autunite is orthorhombic, space group Pnma, Z = 4, a = 14.0135(6), b = 20.7121(8), c = 6.9959(3) [Angstrom], V = 2030.55(15) [[Angstrom].sup.3]. The structure contains the well-known autunite type sheet with composition [[(U[O.sub.2])(P[O.sub.4])].sup.-], resulting from the sharing of equatorial vertices of the uranyl square bipyramids with the phosphate tetrahedra. The calcium atom in the interlayer is coordinated by seven [H.sub.2]O groups and two longer distances to uranyl apical O atoms. Two symmetrically independent [H.sub.2]O groups are held in the structure only by hydrogen bonding. Bond-length-constrained refinement provided a crystal-chemically reasonable description of the hydrogen bonding.
- Published
- 2003