Redetermination of kovdorskite, Mg2PO4(OH)·3H2O

The crystal structure of kovdorskite, ideally Mg2PO4(OH)·3H2O (dimagnesium phosphate hydroxide trihydrate), was reported previously with isotropic displacement paramaters only and without H-atom positions [Ovchinnikov et al. (1980 ▶). Dokl. Akad. Nauk SSSR. 255, 351–354]. In this study, the kovdorskite structure is redetermined based on single-crystal X-ray diffraction data from a sample from the type locality, the Kovdor massif, Kola Peninsula, Russia, with anisotropic displacement parameters for all non-H atoms, with all H-atom located and with higher precision. Moreover, inconsistencies of the previously published structural data with respect to reported and calculated X-ray powder patterns are also discussed. The structure of kovdorskite contains a set of four edge-sharing MgO6 octahedra interconnected by PO4 tetrahedra and O—H⋯O hydrogen bonds, forming columns and channels parallel to [001]. The hydrogen-bonding system in kovdorskite is formed through the water molecules, with the OH− ions contributing little, if any, to the system, as indicated by the long H⋯A distances (>2.50 Å) to the nearest O atoms. The hydrogen-bond lengths determined from the structure refinement agree well with Raman spectroscopic data.


Comment
The pseudo-ternary MgO-P 2 O 5 -H 2 O system has been the subject of numerous studies because magnesium phosphates elicit industrial interest. They are used as bonding in refractories (Kingery, 1950;Lyon et al., 1966) and mortars (Kingery, 1952) or as rapid-setting cements (Sarkar, 1990). They also play an important role in the fertilizer industry due to their solubility properties (Pelly & Bar-On, 1979) and in medical research. In particular, newberyite, Mg(HPO 4 ).3H 2 O, is a constituent of human urinary stones (Sutor et al., 1974) and has been found to act as a self-setting cement for synthetic bone replacements (Klammert et al., 2011) Kovdorskite from the Kovdor massif, Kola Peninsula, Russia was originally described by Kapustin et al. (1980)  In the course of identifying minerals for the RRUFF project (http://rruff.info), we noted that the powder X-ray diffraction pattern of kovdorskite we measured on a sample from the type locality displays some obvious inconsistencies with that calculated from the structure model given by Ovchinnikov et al. (1980) (Fig. 1). For comparison, plotted in Fig. 1 are also the powder X-ray diffraction data tabulated in the original description of the mineral (Kapustin et al., 1980), which clearly agree with our measured data. In seeking the reason behind the discrepancies between the measured and calculated powder X-ray diffraction data and to better understand the relationships between the hydrogen environments and Raman spectra of hydrous minerals, we re-determined the structure of kovdorskite by means of single-crystal X-ray diffraction.
The crystal structure of kovdorskite is characterized by clusters of four edge-sharing MgO 6 octahedra that are interconnected by PO 4 tetrahedra and hydrogen bonds to form columns and channels parallel to [001] (Figs. 2, 3). Within each cluster, there are two special corners where three octahedra are joined. These corners are occupied by hydroxyl ions (OH5).
The hydrogen-bonding system in kovdorskite is mainly formed by the H atoms of H 2 O groups, which are all directed toward supplementary materials sup-2 the channels. The H1 atom (bonded to OH5) contributes little, if any, to the hydrogen bonding system, as indicated by the long H···A distances to the nearest OW6 (2.50 Å) or OH5 (2.51 Å).
An examination of our structure data indicates that the discrepancy in the previously published crystallographic data for kovdorskite (Kapustin et al., 1980;Ovchinnikov et al., 1980;Lake & Craven, 2001) and the mismatch between the measured and calculated powder X-ray diffraction pattern result from the inconsistent choice of the unit-cell settings versus space groups by Kapustin et al. (1980) andOvchinnikov et al. (1980). The space group P2 1 /a and atomic coordinates given by Ovchinnikov et al. (1980) actually correspond to a unit-cell a = 10.45, b =12.90, c = 4.73 Å, and β = 104.3°, which can be derived with the transformation matrix (1 0 1 / 0 1 0 / 0 0 1) from their reported cell parameters. The powder X-ray diffraction pattern calculated using this new unit-cell setting, along with reported space group and atomic coordinates, then matches that  (2), c = 4.7308 (1) Å, and β = 105.054 (1)°, we have space group P2 1 /a. The matrix for the transformation from the former setting to the latter one is the same as that given above. In this study, we have adopted the latter unit-cell setting to facilitate a direct comparison of our atomic coordinates with those reported by Ovchinnikov et al. (1980).
There have been numerous Raman spectroscopic measurements on a variety of phosphates, including barićite, bobierrite (Frost et al., 2002), and newberyite (Frost et al., 2011). Presented in Figure 4 is the Raman spectrum of kovdorskite. A tentative assignment of major Raman bands for this mineral is made according to previous studies on hydrous Mg-phosphate minerals (e.g. Frost et al., 2002Frost et al., , 2011. The most intense, sharp peak at 3681 cm -1 is ascribed to the OH5-H1 stretching mode, whereas three relatively broad bands at 3395, 3219, and 2967 cm -1 are attributable to the O-H stretching vibrations of the H 2 O molecules, and the very broad bump at 1550 ±100 cm -1 to the H 2 O bending vibrations. The O-H···O hydrogen bond lengths inferred from the measured spectrum are in the range 2.62-2.90 Å (Libowitzky, 1999), which compare well with those determined from our X-ray structural analysis (2.65-2.93 Å). Stretching vibrations within the PO 4 group are responsible for the bands between 840 and 1120 cm -1 and bending vibrations for weak bands between 300 and 600 cm -1 .
The bands below 300 cm -1 are attributed to lattice vibrational modes and Mg-O interactions.

Experimental
The kovdorskite specimen used in this study is from the type locality Kovdor Massif, Kola Peninsula, Russia and is in the collection of the RRUFF project (deposition No R050505, http://rruff.info). The chemical composition of the sample was analyzed with a CAMECA SX50 electron microprobe. Only Mg and P, plus very trace amounts of Mn and Ca, were detected. The empirical chemical formula, calculated on the basis of 4.5 O atoms, is Mg 2.00 PO 4.00 (OH).2.67H 2 O, where the amount of H 2 O was estimated by the difference from 100% mass totals.
The Raman spectrum of kovdorskite was collected from a randomly oriented crystal at 100% power on a Thermo Almega microRaman system, using a solid-state laser with a wavenumber of 532 nm, and a thermoelectrically cooled CCD detector.
The laser is partially polarized with 4 cm -1 resolution and a spot size of 1 µm.
supplementary materials sup-3 Refinement All H atoms were located from difference Fourier syntheses and their positions were refined with isotropic displacement parameters. For simplicity, an ideal chemistry, Mg 2.00 PO 4.00 (OH).3H 2 O, was assumed during the final refinement. The highest residual peak in the difference Fourier maps was located at (0.1815, 0.3311, 0.4211), 0.73 Å from O3, and the deepest hole at (0.7791, 0.7157, 0.4322), 0.50 Å from P1. Fig. 1. Comparison of the powder X-ray diffraction patterns for kovdorskite. The patterns are shown vertically offset for clarity: (a) by Kapustin et al. (1980), (b) our measurement, (c) calculated pattern based on the data given by Ovchinnikov et al. (1980), and (d) calculated pattern with space group and atomic coordinates reported by Ovchinnikov et al. (1980), but a transformed unit-cell setting (see text).    (7) 0.00004 (7)