The crystal structure of nacaphite, Na2Ca(PO4)F:: A re-investigation

The crystal structure of nacaphite, Na2Ca(PO4)F, has been re-investigated using an untwinned crystal from a late-stage hydrothermal vein within ijolite-urtite of the Khibiny alkaline massif, Kola Peninsula, Russia. In contrast to previous findings, which identified nacaphite as a pseudo-orthorhombic triclinic mineral, we have shown that nacaphite is monoclinic, P2(1)/c, a 13.3185(14), b 7.0964(8), c 10.6490(11) angstrom, beta 113.526(1)degrees, V 922.81(17) angstrom(3), Z = 8. The structure has been solved by direct methods and refined to R-1 0.034 for 1376 unique observed reflections. It contains two Ca and four Na positions. The Cal site is coordinated by two F- and five circle dot(2-) anions with Ca2+-phi distances in the range of 2.32-2.57 angstrom. The Ca2 cation has six closest anions (two F- and four O2-) with Ca2--phi bond lengths of 2.29-2.37 angstrom, and an additional remote O7 anion at 3.049 angstrom. All Na+ cations are octahedrally coordinated by four O2- and two F- anions each. The Ca2+ and Na2+ cations form cation arrangements parallel to the (100) plane and separated by F- anions and PO43- tetrahedra. The F- ions are coordinated by four Na+ and two Ca2+ cations. The (FNa4Ca2)(7+) octahedra share faces to produce [FNa2Ca](3-) chains parallel to c. The overall topology of the structure is in agreement with the previous studies of the structure. However, there are two essential differences in schemes of order and local geometry of coordination: (1) the Ca2+ and Na+ cations are completely ordered, and the respective sites are fully occupied; (2) the Ca2+ cations have seven-fold coordination of two different types, whereas Na+ cations are octahedrally coordinated. The results of the new crystal-structure refinement of nacaphite are in agreement with the powder X-ray-diffraction pattern of this mineral.