Paraershovite, Na3K3Fe3+2(Si4O10OH)2(OH)2(H 2O)4, a new mineral species from the Khibina Alkaline Massif, Kola peninsula, Russia : description and crystal structure

• Published in: The Canadian Mineralogist
• Main Authors: A. P. Khomyakov, E. Sokolova, N. Yassir Abdu, F. C. Hawthorne, F. Cámara
• Other Authors: A.P. Khomyakov, F.C. Hawthorne
• Format: Article in Journal/Newspaper
• Language: English
• Published: Mineralogical Association of Canada 2010
• Subjects: Geochemistry and Petrology; Settore GEO/06 - Mineralogia; kola peninsula
• Online Access: http://hdl.handle.net/2434/478048; https://doi.org/10.3749/canmin.48.2.279

Paraershovite,Na3K3Fe3+2(Si4O10OH)2(OH)2(H 2O)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), 7γ 1.602(2), 2V(meas.) = 80(3)7deg;, 2V(calc.) = 82°, non-pleochroic, dispersion r > ν (weak), with X Λ c = 84°, Y Λ c = 66°, Z λ c = 24°. Paraershovite is triclinic, space group P1̄, 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)°, V618.46(3J Å3, Z = 1. The strongest lines in the X-ray powderniiffraction pattern [d in A(I)(AW)] are: 11.778(100)(010), 4.390(70)( 11̄1, 02̄1), 3.012(70)(230, 3̄20), 2.606(70)(32̄1,131,012,11̄2), 4.109(60)(1̄11 ,12̄1) and 2.730(60)(32̄1,14̄1,14̄1,31̄1,221). A chemical analysis with an electron microprobe gave Na20 7.77, MgO0.24, Si02 49.67, Al2O30.58, K20 14.07, Fe203 11.29, MnO 0.73,Ti02 3.32, F0.39, H20 10.98,0 = F-0.16, sum 98.88 wt.%; the valence state of Fe was determined by Mössbauer spectroscopy, and the amount of H20 was calculated from the crystal-structure refinement. The empirical formula based on 28 anions is (Na2.44□156)σ4K 2.91 (Fe3+1.38Ti0.40Al 0.11Mn2+0.10Mgi0.06) ∑2.05Si8.05O20 [(OH)3.80 F ...