Abstract
The crystal structures of natural (Mt. Koashva, Khibiny alkaline massif, Kola Peninsula, Russian Arctic) and synthetic (obtained from an aqueous solution of sodium phosphate and sodium fluoride (1:1) by evaporation at room temperature (RT)) natrophosphate, Na7(PO4)2F·19H2O, have been investigated using single-crystal X-ray diffraction analysis. Natrophosphate and its synthetic analogue are cubic, Fd-3c, a = 27.6942(3) Å (natrophosphate at RT), a = 27.6241(4) Å (natrophosphate at 100 K), a = 28.1150(12) Å (synthetic analogue at RT), a = 27.9777(7) Å (synthetic analogue at 100 K). The crystal structure is based upon the super-octahedral [Na6(H2O)18F]5+ polycationic complexes consisting of six edge-linked Na6(OH2)5F octahedra sharing one common fluorine vertex. The A site is statistically occupied by Na and H2O with the prevalence of H2O with the refined occupancy factors O:Na equal to 0.53:0.47 for natrophosphate and 0.75:0.25 for its synthetic analogue. The coordination of the A site in synthetic natrophosphate is enlarged compared to the natural sample, which agrees well with its higher occupancy by H2O molecules. The general formula of natrophosphates can be written as Na6+xHxF(PO4)2·(19 + x)H2O, where x = 0–1. The chemical variability of natrophosphate allows to explain the discrepancies in its solubility reported by different authors. The information-based parameters of structural complexity are equal to 3.713 bit/atom and 2109.177 bit/cell that allows to classify natrophosphate as a structurally very complex mineral.
Highlights
The history of the study of the phase with the chemical formula Na7 (PO4 )2 F·19H2 O is almost 165 years old
Briegleb [1] as transparent octahedral crystals with the composition NaF·Na3 PO4 ·12H2 O. This compound was obtained in the course of soda production by means of the Leblanc process and was studied in 1865 by Rammelsberg [2], who erroneously assigned to it the chemical formula Na3 PO4 ·20H2 O, which was soon corrected by Baumgarten to NaF·2Na3 PO4 ·19H2 O [3]
In agreement with the previous studies [9,10,17], the crystal structure of natrophosphate is based upon the super-octahedral [Na6 (H2 O)18 F]5+ polycationic complexes conF–Na1
Summary
The history of the study of the phase with the chemical formula Na7 (PO4 ) F·19H2 O is almost 165 years old. It was first described in 1856 by H. Briegleb [1] as transparent octahedral crystals with the composition NaF·Na3 PO4 ·12H2 O This compound was obtained in the course of soda production by means of the Leblanc process (later replaced by the Solvay process) and was studied in 1865 by Rammelsberg [2], who erroneously assigned to it the chemical formula Na3 PO4 ·20H2 O, which was soon corrected by Baumgarten to NaF·2Na3 PO4 ·19H2 O [3]. Guiot [7] confirmed these data, whereas Rémy and Guérin [8] proposed the existence of a series of compounds with the general formula
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