Bakhchisaraitsevite, Na2Mg5[PO4](4)center dot 7H(2)O, a new mineral from hydrothermal assemblages related to phoscorite-carbonatite complex of the Kovdor massif, Russia

Bakhchisaraitsevite, ideally Na2Mg5[PO4](4) . 7H(2)O, monoclinic, a = 8.324 (4) Angstrom, b = 12.926(4) Angstrom, c = 17.519(9) Angstrom, beta = 102.03 degrees>(*) over bar * (1), V = 1844 Angstrom (3), space group P2(1)/c, Z = 4 (single crystal diffractometer SYNTEX P (1) over bar, under room temper ature), is a new mineral from the Kovdor massif, Kola alkaline province, Ru ssia. It was found in voids within veins of vuggy dolomite carbonatite that cross-cut carbonatites, phoscorites and alkaline pyroxenites. Associated m inerals include bobierrite, pyrite, collinsite, chlorite, nastrophite, and juonniite. The new mineral occurs as fan-shaped aggregates or single bladed tabular crystals up to 0.5 x 1.5 x 2 mm. It is transparent colourless with white streak, a vitreous lustre, and does not fluoresce under long- or sho rt-wave ultraviolet light. Bakhchisaraitsevite has a Mobs hardness of 2.5, is brittle, cleavage is perfect on {001}. The measured density is 2.50(2) g /cm(3), calculated - 2.47(4) g/cm(3). It is biaxial, close to neutral, with alpha = 1.538(1), beta = 1.540(1), gamma = 1.543(1), 2V(calc.) = 72.5 degr ees, b = gamma, C : alpha = 45 +/- 3 degrees, no twinning was observed. The X-ray powder diffraction pattern shows strong reflections at [d(Angstrom), (I), (hkl)]: 10.31(33)(011), 8.56(100)(002), 3.496(23) (-124), 3.020(28)(- 125), 2.849(33)(231). Electron probe analysis gave (wt.%): 9.17 Na2O, 29.40 MgO, 0.33 MnO, 0.84 FeO, 0.07 CaO, 41.57 P2O5; 18.62 H2O (added for total of 100 wt.%). The empirical formula is (Na2.02Ca0.01)(Sigma2.03)(Mg4.98Fe2+ Mn-0.08(0.03) )(Sigma5.09)P(4)O(16.11)7.06H(2)O. The IR spectrum of the new mineral inclu des following bands (in cm(-1), the strongest are underlined): 216, 268, 28 2, 344, 446, 476, 554, 586, 610, 670, 790 848, 948, 956, 980, 1024, 1040, 1 080, 1118, 1460, 1674, 3140, 3370. The mineral is named in honour of a mine ralogist from Kola Sci. Centre, ALEXANDER YU. BAKHCHI-SARAITSEV (1947-1998) . According to the similarity of X-ray patterns and cell parameters, stoich ometry and IR-spectrums, bakhchisaraitsevite is close to another hydrotherm al phosphate from Kovdor, rimkorolgite, and to the artificial alkaline phos phate, synthesised during microbial sulphate-reducing experiments in marine sediments. Bakhchisaraitsevite is supposed to occur as a possible biominer al, accumulating phosphorus in sulphide-bearing reduced sediments reworked by bacteria.