Sveinbergeite, Ca(Fe²⁺₆Fe³⁺)Ti₂(Si₄O₁₂)₂O₂(OH)₅(H₂O)₄, a new astrophyllite-group mineral from the Larvik Plutonic Complex, Oslo Region, Norway: description and crystal structure

Author： Khomyakov A. P. Cámara F. Sokolova E. Abdu Y. Hawthorne F. C.

Publisher： Mineralogical Society

ISSN： 0026-461X

Source： Mineralogical Magazine, Vol.75, Iss.5, 2011-10, pp. : 2687-2702

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Abstract

Sveinbergeite, Ca(Fe²⁺₆Fe³⁺)Ti₂(Si₄O₁₂)₂O₂(OH)₅(H₂O)₄, is a new astrophyllite-group mineral discovered in a syenite pegmatite at Buer on the Vesterøya peninsula, Sandefjord, Oslo Region, Norway. The mineral occurs in pegmatite cavities as 0.01–0.05 mm thick lamellar (0.2–0.5 × 5–10 mm) crystals forming rosette-like divergent groups and spherical aggregates, which are covered by brown coatings of iron (and possibly manganese) oxides, associated with magnesiokatophorite, aegirine, microcline, albite, calcite, fluorapatite, molybdenite, galena and a hochelagaite-like mineral. Crystals of sveinbergeite are deep green with a pale green streak and a vitreous and pearly lustre. Sveinbergeite has perfect cleavage on {001} and a Mohs hardness of 3. Its calculated density is 3.152 g/cm³. Itis biaxial positive with α 1.745(2), β 1.746(2), γ 1.753(2), 2V(meas.) = 20(3)°. The mineral is pleochroic according to the scheme Z > X ∼ Y : Z is deep green, X and Y are brownish green. Orientation is as follows: X ⊥ (001), Y ˆ b = 12°, Z = a, elongation positive. Sveinbergeite is triclinic, space group P1, a = 5.329(4), b = 11.803(8), c = 11.822(8) Å; α = 101.140(8)°, β = 98.224(8)°, γ = 102.442(8)°; V = 699.0(8) Å_3;Z = 1. The nine strongest lines in the X-ray powder diffraction pattern [d in Å (I)(hkl)] are: 11.395(100)(001,010), 2.880(38)(004), 2.640(31)(210,141), 1.643(24)(071,072), 2.492(20)(211), 1.616(15)(070), 1.573(14)(322), 2.270(13)(134) and 2.757(12)(140,132). Chemical analysis by electron microprobe gave Nb₂O₅0.55, TiO₂ 10.76, ZrO₂ 0.48, SiO₂ 34.41, Al₂O₃ 0.34, Fe₂O₃ 5.57, FeO 29.39, MnO 1.27, CaO 3.87, MgO 0.52, K₂O 0.49, Na₂O 0.27, F 0.24, H₂O 8.05, O=F – 0.10, sum 96.11 wt.%, the amount of H₂O was determined from structure refinement, and the valence state of Fe was calculated from structure refinementin accord with Mössbauer spectroscopy. The empirical formula, calculated on the basis of eight(Si + Al) p.f.u., is (Ca_0.95Na_0.12K_0.14)_{S 1.21}(Fe²⁺_5.65Fe³⁺_0.93Mn_0.25Mg_0.18)_Σ7.01(Ti_1.86Nb_0.06Zr_0.05Fe³⁺_0.03)_Σ2(Si_7.91Al_0.09)_Σ8O_34.61H_12.34F_0.17, Z = 1. The infrared spectrum of the mineral contains the following absorption frequencies: 3588, ∼3398 (broad), ∼3204 (broad), 1628, 1069, 1009, 942, 702, 655 and 560 cm^–1. The crystal structure of the mineral was solved by direct methods and refined to an R₁ index of 21.81%. The main structural unit in the sveinbergeite structure is an HOH layer which is topologically identical to that in the astrophyllite structure. Sveinbergeite differs from all other minerals of the astrophyllite group in the composition and topology of the interstitial A and B sites and linkage of adjacent HOH layers. The mineral is named in honour of Svein Arne Berge (b. 1949), a noted Norwegian amateur mineralogist and collector who was the first to observe and record this mineral from its type locality as a potential new species.