Middendorfite, K3Na2Mn5Si12(O,OH)36 · 2H2O, a new mineral species from the Khibiny pluton, Kola Peninsula

Abstract

Middendorfite, a new mineral species, has been found in a hydrothermal assemblage in Hilairite hyperperalkaline pegmatite at the Kirovsky Mine, Mount Kukisvumchorr apatite deposit, Khibiny alkaline pluton, Kola Peninsula, Russia. Microcline, sodalite, cancrisilite, aegirine, calcite, natrolite, fluorite, narsarsukite, labuntsovite-Mn, mangan-neptunite, and donnayite are associated minerals. Middendorfite occurs as rhombshaped lamellar and tabular crystals up to 0.1 × 0.2 × 0.4 mm in size, which are combined in worm-and fanlike segregations up to 1 mm in size. The color is dark to bright orange, with a yellowish streak and vitreous luster. The mineral is transparent. The cleavage (001) is perfect, micalike; the fracture is scaly; flakes are flexible but not elastic. The Mohs hardness is 3 to 3.5. Density is 2.60 g/cm3 (meas.) and 2.65 g/cm3 (calc.). Middendorfite is biaxial (−), α = 1.534, β = 1.562, and γ = 1.563; 2V (meas.) = 10°. The mineral is pleochroic strongly from yellowish to colorless on X through brown on Y and to deep brown on Z. Optical orientation: X = c. The chemical composition (electron microprobe, H2O determined with Penfield method) is as follows (wt %): 4.55 Na2O, 10.16 K2O, 0.11 CaO, 0.18 MgO, 24.88 MnO, 0.68 FeO, 0.15 ZnO, 0.20 Al2O3, 50.87 SiO2, 0.17 TiO2, 0.23 F, 7.73 H2O; −O=F2−0.10, total is 99.81. The empirical formula calculated on the basis of (Si,Al)12(O,OH,F)36 is K3.04(Na2.07Ca0.03)Σ2.10(Mn4.95Fe0.13Mg0.06Ti0.03Zn0.03)Σ5.20(Si11.94Al0.06)Σ12O27.57(OH)8.26F0.17 · 1.92H2O. The simplified formula is K3Na2Mn5Si12(O,OH)36 · 2H2O. Middenforite is monoclinic, space group: P21/m or P21. The unit cell dimensions are a = 12.55, b = 5.721, c = 26.86 A; β = 114.04°, V = 1761 A3, Z = 2. The strongest lines in the X-ray powder pattern [d, A, (I)(hkl)] are: 12.28(100)(002), 4.31(81)(11 $$\overline 4 $$ ), 3.555(62)(301, 212), 3.063(52)(008, 31 $$\overline 6 $$ ), 2.840(90)(312, 021, 30 $$\overline 9 $$ ), 2.634(88)(21 $$\overline 9 $$ , 1.0. $$\overline 1 $$ 0, 12 $$\overline 4 $$ ), 2.366(76)(22 $$\overline 6 $$ , 3.1. $$\overline 1 $$ 0, 32 $$\overline 3 $$ ), 2.109(54)(42–33, 42–44, 51 $$\overline 9 $$ , 414), 1.669(64)(2.2. $$\overline 1 $$ 3, 3.2. $$\overline 1 $$ 3, 62 $$\overline 3 $$ , 6.1. $$\overline 1 $$ 3), 1.614(56)(5.0. $$\overline 1 $$ 6, 137, 333, 71 $$\overline 1 $$ ). The infrared spectrum is given. Middendorfite is a phyllosilicate related to bannisterite, parsenttensite, and the minerals of the ganophyllite and stilpnomelane groups. The new mineral is named in memory of A.F. von Middendorff (1815–1894), an outstanding scientist, who carried out the first mineralogical investigations in the Khibiny pluton. The type material of middenforite has been deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.