Titre du document / Document title

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

Auteur(s) / Author(s)

KAMPF Anthony R. ; MERLINO Stefano ; PASERO Marco ;

Résumé / Abstract

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 C1, a = 7.722(3), b = 7.516(3), c = 12.206(4) Å, a = 98.86(1), β = 96.91(1), γ = 90.00(1)°, V = 694.8(3) Å₃, Z = 4, yielding R = 5.1% for 1420 reflections with F^o > 4σ(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, β= 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, a = 98.86, β = 96.91, γ = 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.

Revue / Journal Title

The American mineralogist   ISSN 0003-004X   CODEN AMMIAY 

Source / Source

2003, vol. 88, n^o2-3, pp. 430-435 [6 page(s) (article)]

Langue / Language

Anglais

Editeur / Publisher

Mineralogical Society of America, Washington, DC, ETATS-UNIS  (1916) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 503, 35400010415694.0190