Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Kampf, A. R. Articles by Pasero, M. Search for Related Content GeoRef GeoRef Citation

Order-disorder approach to calcioaravaipaite, [PbCa₂Al(F,OH)₉]: The crystal structure of the triclinic MDO polytype

¹ Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, California 90007, U.S.A.
² Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I-56126 Pisa, Italy

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

The crystal structure of calcioaravaipaite, PbCa₂Al(F,OH)₉, was initially solved by direct methods in the monoclinic space group A2/m (R = 12.4%). Further study demonstrated the OD nature of the structure, and showed that the crystal was twinned. The structure was solved in the triclinic space group C, a = 7.722(3), b = 7.516(3), c = 12.206(4) Å, = 98.86(1), ß = 96.91(1), = 90.00(1)°, V = 694.8(3) ų, Z = 4, yielding R = 5.1% for 1420 reflections with F_o > 4(F_o).

Calcioaravaipaite belongs to a family of order-disorder (OD) structures formed by equivalent layers of symmetry C2/m. Two maximum-degree-of-order (MDO) polytypes are possible. MDO1 results from a regular alternation of stacking operators 2_1/2 and 2_–1/2 and yields a monoclinic structure with C2/c, a = 7.72, b = 7.52, c = 24.12 Å, ß = 96.99°. MDO2 results from the sequence 2_1/2 / 2_1/2 / 2_1/2 /... and yields a triclinic structure with a = 7.72, b = 7.52, c = 12.21 Å, = 98.86, ß = 96.91, = 90.00°.

The structure of calcioaravaipaite is comprised of two kinds of alternating polyhedral slabs parallel to (001). Slab 1 consists of a fluorite-like double layer of edge-sharing (CaF₈) distorted cubes and slab 2 is a composite of face- and edge-sharing (PbF₁₂) polyhedra and outlying (AlF₆) octahedra, the latter sharing faces and edges with the (PbF₁₂) polyhedra, but no elements with one another. Aravaipaite and calcioaravaipaite share a common fluorite-type layer; however, in aravaipaite the presence of Pb²⁺ rather than Ca²⁺ in this layer results in slabs of strikingly different polyhedral configuration.

This article has been cited by other articles:

				G. Ventruti, F. Scordari, E. Schingaro, A. F. Gualtieri, and C. Meneghini The order-disorder character of FeOHSO4 obtained from the thermal decomposition of metahohmannite, Fe3+2(H2O)4[O(SO4)2] American Mineralogist, April 1, 2005; 90(4): 679 - 686. [Abstract] [Full Text] [PDF]