PARAERSHOVITE, Na3K3Fe3+2(Si4O10OH)2(OH)2(H2O)4, A NEW MINERAL SPECIES FROM THE KHIBINA ALKALINE MASSIF, KOLA PENINSULA, RUSSIA: DESCRIPTION AND CRYSTAL STRUCTURE

Abstract

Paraershovite, Na 3 K 3 Fe 3+ 2 (Si 4 O 10 OH) 2 (OH) 2 (H 2 O) 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/cm 3 ) 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), 2 V (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 P 1, a 10.1978(5), b 12.0155(6), c 5.2263(3) A, α 103.439(1), β 96.020(1), γ 91.683(1)°, V 618.46(3) A 3 , Z = 1. The strongest lines in the X-ray powder-diffraction pattern [ d in A(I)( hkl )] are: 11.778(100)(010), 4.390(70)(111, 021), 3.012(70)(230, 320), 2.606(70)(321, 131, 012, 112), 4.109(60)(111, 121) and 2.730(60)(321, 141, 141, 311, 221). A chemical analysis with an electron microprobe gave Na 2 O 7.77, MgO 0.24, SiO 2 49.67, Al 2 O 3 0.58, K 2 O 14.07, Fe 2 O 3 11.29, MnO 0.73, TiO 2 3.32, F 0.39, H 2 O 10.98, O = F –0.16, sum 98.88 wt.%; the valence state of Fe was determined by Mossbauer spectroscopy, and the amount of H 2 O was calculated from the crystal-structure refinement. The empirical formula based on 28 anions is (Na 2.44 □ 1.56 ) ∑4 K 2.91 (Fe 3+ 1.38 Ti 0.40 Al 0.11 Mn 2+ 0.10 Mg 0.06 ) ∑2.05 Si 8.05 O 20 [(OH) 3.80 F 0.20 ] ∑4 (H 2 O) 4 . The IR spectrum of the mineral has absorption bands at 1600 and 3500 cm −1 , showing the presence of H 2 O and OH groups in the structure. The crystal structure of paraershovite was solved by direct methods and refined to an R 1 index of 4.28% (for 2134 observed [ F o > 4σ F ] unique reflections measured with Mo K α radiation on a Bruker P 4 diffractometer with a CCD 4K APEX detector). The crystal structure of paraershovite is isostructural with that of ershovite, ideally Na 4 K 3 (Fe 2+ ,Mn,Ti) 2 (Si 4 O 10 OH) 2 (OH) 2 (H 2 O) 4 ( a 10.244, b 11.924, c 5.276 A, α 103.491, β 96.960, γ 91.945°, V 620.8 A 3 , space group P 1). Paraershovite is a Na-deficient, Fe 3+ -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 □ + Fe 3+ 2 → Na + + M 2.5+ 2 , where M 2.5+ represents Fe 2+ , Mn 2+ and Ti 4+ . The name paraershovite , derived from the Greek para (close by) and ershovite , recalls the close structural, chemical and genetic relations between paraershovite and ershovite.