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-1, a = 7.722(3), b = 7.516(3), c = 12.206(4) Å, α = 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 F0 > 4σ(F0). 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 21/2 and 21/2 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 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 avaraipaite the presence of Pb2+ rather than Ca2+ in this layer results in slabs of strikingly different polyhedral configuration.
|Autori interni:||MERLINO, STEFANO|
|Autori:||KAMPF AR; MERLINO S; PASERO M|
|Titolo:||OD approach to calcioaravaipaite, [PbCa2Al(F,OH)9]: the crystal structure of the triclinic MDO polytype|
|Anno del prodotto:||2003|
|Appare nelle tipologie:||1.1 Articolo in rivista|