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THE CRYSTAL STRUCTURE OF NACAPHITE, Na₂Ca(PO₄)F: A RE-INVESTIGATION

Sergey V. Krivovichev^1,, Victor N. Yakovenchuk², Gregory Yu. Ivanyuk², Yakov A. Pakhomovsky², Thomas Armbruster³ and Ekaterina A. Selivanova⁴

¹ Department of Crystallography, St. Petersburg State University, University Emb. 7/9, St. Petersburg 199034, Russia
² Geological Institute, Kola Science Center, Russian Academy of Sciences, Apatity 184209, Russia
³ Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, CH–3102 Bern, Switzerland
⁴ Geological Institute, Kola Science Center, Russian Academy of Sciences, Apatity 184209, Russia

E-mail address: skrivovi{at}mail.ru

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, P2₁/c, a 13.3185(14), b 7.0964(8), c 10.6490(11) Å, β 113.526(1)°, V 922.81(17) ų, Z = 8. The structure has been solved by direct methods and refined to R¹ 0.034 for 1376 unique observed reflections. It contains two Ca and four Na positions. The Ca1 site is coordinated by two F^– and five O^2– anions with Ca²⁺– distances in the range of 2.32–2.57 Å. The Ca2 cation has six closest anions (two F^– and four O^2–) with Ca²⁺ – bond lengths of 2.29–2.37 Å, and an additional remote O7 anion at 3.049 Å. All Na⁺ cations are octahedrally coordinated by four O^2– and two F^– anions each. The Ca²⁺ and Na²⁺ cations form cation arrangements parallel to the (100) plane and separated by F^– anions and PO₄^3– 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.

Keywords: nacaphite, crystal structure, twinning, Kola Peninsula, Russia.