Nacaphite: re-investigazione della struttura cristallina

[Marco E. Ciriotti]

La struttura cristallina della nacaphite è stata re-investigata. L'articolo sarà pubblicato in un prossimo numero del Canadian Mineralogist.

Referenza:
• Krivovichev, S.V., Yakovenchuk, V.N., Ivanyuk, G.Yu., Pakhomovsky, Ya.A., Armbruster, T., Selivanova, E.A. (2007): The crystal structure of nacaphite, Na2Ca(PO4)F: a re-investigation. Canadian Mineralogist, 45, (in stampa).

[Marco E. Ciriotti]

Referenza:
• Krivovichev, S.V., Yakovenchuk, V.N., Ivanyuk, G.Yu., Pakhomovsky, Ya.A., Armbruster, T., Selivanova, E.A. (2007): The crystal structure of nacaphite, Na2Ca(PO4)F: a re-investigation. Canadian Mineralogist, 45, 915-920.

Abstract:
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, ß 113.526(1)°, V 922.81(17) Å3, 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 O2– anions with Ca2+– distances in the range of 2.32–2.57 Å. The Ca2 cation has six closest anions (two F– and four O2–) 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 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.

[Marco E. Ciriotti]

In seguito al raffinamento della struttura (vedasi messaggio precedente), la formula della nacaphite è ora la seguente: Na2Ca(PO4)F.

La nacaphite forma una serie polisomatica con la seidozerite, anch'essa monoclina.

[Marco E. Ciriotti]

Referenza:
▪ Avdontceva, M.S., Krzhizhanovskaya, M.G., Krivovichev, S.V., Zolotarev, A.A. Yakovenchuk, V.N. (2022): Polymorphism of Na2CaPO4F: Crystal structures, thermal stability and structural complexity. Journal of Solid State Chemistry, 318, (in press).

Abstract:
The compound Na2CaPO4F has at least three polymorphic modifications: monoclinic α (also known as the natural mineral nacaphite), orthorhombic β and rhombohedral γ. All three polymorphs have antiperovskite-type structures belonging to either 2H (α and β) or 15R (γ) polytypes. The β-phase was synthesized using CaF2, NaF and Na3PO4 as initial reagents at 800 °C. Its crystal structure is orthorhombic (Pnma, a = 5.3542(1), b = 7.0878(2), c = 12.2560(3) Å, V = 465.11(3) ų, Z = 4) and based upon the chains of fluorine-centered face-sharing octahedra running along [100]. Upon heating, the β form is stable up to 640 °C, when it melts and, in the temperature range of 640–800 °C, the γ form crystallizes. Its crystal structure (rhombohedral, R3¯m, a = 7.0272(3), c = 40.609(2) Å, V = 1736.66(18) ų, Z = 15) consists of framework based upon pentamers of face-sharing [F(Na,Ca)6] octahedra connected to each other through common Na vertices. The strongest thermal expansion for both modifications is parallel to the modules of face-sharing anion-centered octahedra, whereas it is almost isotropic within the plane perpendicular to the modules. The information-theoretic structural complexity analysis points out to the possible metastable character of the β-polymorph. The proposed stability row of the Na2CaPO4F phases under ambient conditions corresponds to the following sequence: α > γ > β. This agrees well with the relations among their structural complexities, degrees of order, physical and information densities. The sequence of phase transitions α → β → γ proceeds via transitional metastable β-phase. The α → β transition is reversible and of the order-disorder type with the conservation of structural topology, whereas the β → γ transition is reconstructive and irreversible. The latter transition is associated with the transformation of the antiperovskite 2H polytype into the 15R polytype.

[Marco E. Ciriotti]

Referenza:
▪ Avdontceva, M.S., Krzhizhanovskaya, M.G., Krivovichev, S.V., Zolotarev, A.A. Yakovenchuk, V.N. (2022): Polymorphism of Na2CaPO4F: Crystal structures, thermal stability and structural complexity. Journal of Solid State Chemistry, 318, 123779.