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Sveinbergeite, Ca(Fe2+6 Fe3+)Ti2(Si4O12)2O2(OH)5(H2O)4, a new astrophyllite-group mineral from the Larvik Plutonic Complex, Oslo Region, Norway: description and crystal structure
- A. P. Khomyakov, F. Cámara, E. Sokolova, Y. Abdu, F. C. Hawthorne
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- Journal:
- Mineralogical Magazine / Volume 75 / Issue 5 / October 2011
- Published online by Cambridge University Press:
- 05 July 2018, pp. 2687-2702
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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 Vesteroya 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. It is 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 ┴ L (001), Y ᶺ b = 12°, Z = a, elongation positive. Sveinbergeite is triclinic, space group P1̄, 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) Å3; 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)(2̄10,l̄41), 1.643(24)(07̄1,072), 2.492(20)(2l̄l), 1.616(15)(070), 1.573(14)(3̄2̄2), 2.270(13)(1̄3̄4) and 2.757(12)(1̄40,1̄3̄2). Chemical analysis by electron microprobe gave Nb2O5 0.55, TiO2 10.76, ZrO2 0.48, SiO2 34.41, A12O3 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 refinement in accord with Mossbauer spectroscopy. The empirical formula, calculated on the basis of eight (Si + Al) p.f.u., is (Ca0.95Na0.12K0.14)Σ1.21(Fe2+5.65Fe3+0.93Mn0.25Mg0.18)Σ7.01(Ti1.86Nb0.06Zr0.05Fe3+)Σ2 (Si7091Al0.09)Σ8O34.61H12.34F0.17, 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.
Further developments in the structure topology of the astrophyllite-group minerals
- E. Sokolova
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- Journal:
- Mineralogical Magazine / Volume 76 / Issue 4 / August 2012
- Published online by Cambridge University Press:
- 05 July 2018, pp. 863-882
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The structure topology and crystal chemistry have been considered for ten astrophyllite-group minerals that contain the HOH layer, a central trioctahedral (O) sheet and two adjacent (H) sheets of [5]- and [6]-coordinated D polyhedra and the astrophyllite (T4O12) ribbons. The HOH layer is characterized by a planar cell with a ∼5.4, b ∼11.9 A ˚ and a^b ∼103°. The ideal composition of the O sheet is Fe72+ (astrophyllite) or Mn72+(kupletskite). All structures consist of an HOH layer and an I (intermediate) block that consists of atoms between two HOH layers. In the astrophyllite group, there are two types of structures based on the type of linkage of HOH layers: (1) HOH layers link directly where they share common vertices of D octahedra, and (2) HOH layers do not link directly via polyhedra of the H sheets. The type-1 structure occurs in astrophyllite, niobophyllite, nalivkinite, tarbagataite, kupletskite, niobokupletskite and kupletskite-(Cs); the type-2 structure occurs in magnesioastrophyllite, sveinbergeite and devitoite. The general formulae for the eight astrophyllite-group minerals (astrophyllite, niobophyllite, nalivkinite, tarbagataite, kupletskite, niobokupletskite, kupletskite-(Cs), magnesioastrophyllite) and for the extended astrophyllite group including devitoite and sveinbergeite are A2BC7D2T8O26(OH)4X0–1 and A2pBrC7D2(T4O12)2IXD2OXA4OXDnP, respectively, where C and D are cations of the O and H sheets, C =[6](Fe2+, Mn, Fe3+, Na, Mg, Zn) at the M(1–4) sites; D = [6,5](Ti, Nb, Zr, Fe3+); T = Si, minor Al; A2pBrI is the composition of the I block where p = 1,2; r = 1,2; A = K, Cs, Li, Ba, H2O, ☐; B = Na, Ca, Ba, H2O, ☐; I represents the composition of the central part of the I block, excluding peripheral layers of the form A2B; X = O, OH, F and H2O; n = 0, 1, 2. Two topological issues have been considered: (1) the pattern of sizes of the M octahedra in the O sheet, M(1) > M(2) > M(3) > M(4) and (2) different topologies of the HOH layer in magnesioastrophyllite and all other structures of the astrophyllite group.