Modular approach in solving the crystal structure of a synthetic dimorph of nacaphite, Na2Ca[PO4]F, from powder-diffraction data

By comparison with the known structures of nacaphite and arctite, the cryst al structure of a synthetic dimorph of nacaphite, (Na0.67Ca0.33)(3)PO4F, ha s been solved using a modular approach. The structure model, refined by Rie tveld method on the basis of X-ray powder-diffraction data including minor quantities of villiaumite, fluorite and fluorapatite, consists of a stackin g of 15 nacaphite-type 7.1 x 7.1 x 2.7 Angstrom modules (slabs) along the [ 001] direction of a rhombohedral R3m cell with a = 7.0179(7), c = 40.56(4) Angstrom. Nacaphite, on the other hand, is triclinic and is based on the st acking of only four such slabs. The slabs and their links in nacaphite and its synthetic dimorph are quite different in detail; thus the two compounds are polymorphs and not polytypes: The role of nacaphite modules in buildin g minerals of hyperagpaitic rocks is discussed in terms of merotype and ple siotype series. Modular relationships with sulphohalite-type minerals also are discussed.