Paraershovite, Na3K3Fe3+2 (Si4O10OH)2(OH)2(H2O)4, a new mineral species from the Khibina alkaline massif, Kola peninsula, Russia: mineral description and crystal structure

Paraershovite, Na3K3Fe3+2 (Si4O10OH)2(OH)2(H2O)4, is a new silicate mineral species discovered in a hyperagpaitic pegmatite from Mount Yukspor, Khibina alkaline massif, Kola Peninsula, Russia. The mineral occurs as equant (100) platy and [001] elongate prismatic crystals up to 0.5–1 mm and aggregates up to 2–3 mm sporadically scattered in a pegmatite matrix. Associated minerals are sodalite, aegirine, arfvedsonite, pectolite, shcherbakovite, lamprophyllite, lomonosovite, shafranovskite, villiaumite and natrophosphate. Crystals of paraershovite are yellow with orange or pinkish shades, with a white streak and vitreous luster. The mineral is translucent or transparent, and non-fluorescent under 240–400 nm ultraviolet radiation. Paraershovite has a perfect {100} cleavage. It is brittle, has a step-like and splintery fracture, and a Mohs hardness of 3. Its observed and calculated densities (g/cm3) are 2.60(3) (by microvolumetric methods) and 2.584 (using the empirical formula), respectively. It is biaxial positive, with α 1.569(2), β 1.583(2), γ 1.602(2), 2V(meas.) = 80(3)°, 2V(calc.) = 82°, non-pleochroic, dispersion r > v (weak), with X ∧ c = 84°, Y ∧ c = 66°, Z ∧ c = 24°. Paraershovite is triclinic, space group P1̅, a 10.1978(5), b 12.0155(6), c 5.2263(3) Å, α 103.439(1), β 96.020(1), γ 91.683(1)°, V 618.46(3) Å3, Z = 1. The strongest lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 11.778(100)(010), 4.390(70)(11̅1, 02̅1), 3.012(70)(230, 3̅20), 2.606(70)(32̅1, 131, 01̅2, 1̅1̅2), 4.109(60)(1̅11, 1̅2̅1) and 2.730(60)(3̅2̅1, 1̅4̅1, 14̅1, 31̅1, 221). A chemical analysis with an electron microprobe gave Na2O 7.77, MgO 0.24, SiO2 49.67, Al2O3 0.58, K2O 14.07, Fe2O3 11.29, MnO 0.73, TiO2 3.32, F 0.39, H2O 10.98, O = F –0.16, sum 98.88 wt.%; the valence state of Fe was determined by Mössbauer spectroscopy, and the amount of H2O was calculated from the crystal-structure refinement. The empirical formula based on 28 anions is (Na2.44□1.56)∑4K2.91 (Fe3+1.38Ti0.40Al0.11Mn2+0.10 Mg0.06)∑2.05Si8.05O20 [(OH)3.80 F0.20]∑4 (H2O)4. The IR spectrum of the mineral has absorption bands at 1600 and 3500 cm−1, showing the presence of H2O and OH groups in the structure. The crystal structure of paraershovite was solved by direct methods and refined to an R1 index of 4.28% (for 2134 observed [Fo > 4σF] unique reflections measured with MoKα radiation on a Bruker P4 diffractometer with a CCD 4K APEX detector). The crystal structure of paraershovite is isostructural with that of ershovite, ideally Na4K3(Fe2+,Mn,Ti)2 (Si4O10OH)2(OH)2(H2O)4 (a 10.244, b 11.924, c 5.276 Å, α 103.491, β 96.960, γ 91.945°, V 620.8 Å3, space group P1̅). Paraershovite is a Na-deficient, Fe3+-dominant analogue of ershovite. It can be considered an oxidized product of the epithermal alteration of ershovite. Paraershovite is chemically related to ershovite by the heterovalent substitution □ + Fe3+2 → Na+ + M2.5+2, where M2.5+ represents Fe2+, Mn2+ and Ti4+. The name paraershovite, derived from the Greek para (close by) and ershovite, recalls the close structural, chemical and genetic relations between paraershovite and ershovite.