PAKHOMOVSKYITE, CO3(PO4)2 •8H2O, A NEW MINERAL SPECIES FROM KOVDOR, KOLA PENINSULA, RUSSIA

Abstract

Pakhomovskyite, Co3(PO4)2 •8H2O, is a new mineral species found in dolomite carbonatite cutting phoscorites in the Iron-Ore Complex, Kovdor, Kola Peninsula, Russia. The mineral was found as groups (up to 3 × 2 cm) of small spherulites (up to 0.5 mm diameter) and rosettes (up to 0.05 mm diameter) of well-shaped tabular crystals growing on walls of leached fissures in dolomitic carbonatite. The associated minerals are bakbchisaraitsevite, bobierrite, magnetite, kovdorskite, rimkorolgite, juonniite, norsethite, chalcopyrite, phlogopite, pyrrhotite and pyrite. Pakhomovskyite is bright pink, with a dull luster in aggregates, and pearly luster in separate flakes. The streak is pink. The spherulites are translucent, non-fluorescent. The Mohs hardness is about 2. The mineral is flexible in thin flakes. Cleavage is perfect on {010}, and the fracture is stepped. Density is 2.71(2) g/cm3 (measured using heavy liquids) and 2.71 g/cm3 (calculated). Pakhomovskyite is biaxial (+), α 1.581(2), β 1.600(2), γ 1.631(2) (589 nm), 2V (meas.) 75-80°, 2V (calc.) 77°. The dispersion is weak, r < v. Optical orientation: X = b, Y Λ c in the range 22-23° in the obtuse β angle, with pleochroism from pale pink on Y to pinkish grey on Z. Chemical analysis with an electron microprobe gave (wt%): CoO 34.88, MgO 2.97, MnO 2.41, FeO 0.40, NiO 0.53, P2O5 27.95, H2O (by the Penfield method) 29.50, total 98.64. The empirical formula calculated on the basis of Co+Mg+Mn+Ni+Fe+P = 5 is (Co2.38Mg0.38 Mn0.17Ni0.04 Fe2+0.03)∑2.99 (PO4)2.01•8.35H2O. The simplified formula is Co3(PO4)2 •8H2O. According to X-ray-diffraction studies, pakhomovskyite is monoclinic, C2/m, a 10.034(4), b 13.341(3), c 4.670(3) Å, β 105.02(2)°, V 603.8 Å3, Z = 2. The strongest six lines in the X-ray powder-diffraction pattern [dobs(Å)(Iobs)hkl] are: 6.67(100)(020), 3.195(60)(131̄), 2.948(70)(311̄), 2.691(70)(221), 2.521(60) (241̄), 2.408(60)(401̄). Pakhomovskyite, a new member of the vivianite group, is an analogue of synthetic Co3 (PO4)2•8H2O. Pakhomovskyite is the latest low-temperature hydrothermal mineral to form, as a result of dissolution of primary ore minerals by alkaline phosphate-bearing solutions. The mineral is named in honor of Yakov A. Pakhomovsky, who has made significant contributions to the mineralogy of the alkaline massifs of the Kola Peninsula.