Order-disorder approach to calcioaravaipaite, [PbCa 2 Al(F,OH) 9 ]: The crystal structure of the triclinic MDO polytype

The crystal structure of calcioaravaipaite, PbCa 2 Al(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 C1̅ , 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) Å 3 , 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, α = 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 (CaF 8 ) distorted cubes and slab 2 is a composite of face- and edge-sharing (PbF 12 ) polyhedra and outlying (AlF 6 ) octahedra, the latter sharing faces and edges with the (PbF 12 ) polyhedra, but no elements with one another. Aravaipaite and calcioaravaipaite share a common fluorite-type layer; however, in aravaipaite the presence of Pb 2+ rather than Ca 2+ in this layer results in slabs of strikingly different polyhedral configuration.