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The crystal structure of aravaipaite

Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, U.S.A.

Correspondence: ^* E-mail: akampf{at}nhm.org

The crystal structure of aravaipaite, Pb₃AlF₉·H₂O, monoclinic, P2₁/n, a = 25.048(4), b = 5.8459(8), c = 5.6805(7) Å, ß = 94.013(3)°, V = 829.7(2) ų, Z=4, was solved by direct methods and refined by full-matrix least-squares techniques to R = 0.049 for 1170 observed reflections [F_o > 4(F_o)] and R = 0.089 for all 1820 reflections collected using MoK X-radiation and a CCD-based detector. The structure of aravaipaite contains a square-packed layer of F atoms on either side of which are bonded Pb atoms in a fluorite (ß-PbF₂)-type configuration. This layer parallel to {100} serves as a template to which on both sides are attached AlF₆ octahedra and PbF₆(H₂O)₂ polyhedra. The resulting thick slabs are connected via Pb-O-Pb and Al-F-Pb bonds. The two nonequivalent Pb atoms in the fluorite-type layer are each coordinated to 11 F atoms and exhibit typical lone-pair behavior.

Aravaipaite was originally reported to be triclinic. The structure analysis yielded the new monoclinic cell provided above and required the following revisions in the mineral’s description. Morphology: forms {100} and {401}; lamellar on {100}. Twinning: polysynthetic on {100}. Cleavage: {100} perfect micaceous, {011} good, {010} and {001} fair. Density (calc.): 6.703 g/cm³. Optical orientation: X = b; Z a = 24° in the obtuse angle ß. A powder pattern calculated from the structure data is also provided.

This article has been cited by other articles:

				A. R. Kampf, A. R. Kampf, S. Merlino, and M. Pasero Order-disorder approach to calcioaravaipaite, [PbCa2Al(F,OH)9]: The crystal structure of the triclinic MDO polytype American Mineralogist, February 1, 2003; 88(2-3): 430 - 435. [Abstract] [Full Text] [PDF]