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A new anhydrous amphibole from the Eifel region, Germany: Description and crystal structure of obertiite, NaNa2(Mg3Fe3+Ti4+)Si8O22O2

  • Frank C. Hawthorne EMAIL logo , Mark A. Cooper , Joel D. Grice and Luisa Ottolini
From the journal American Mineralogist

Abstract

Obertiite is a new amphibole species from Bellerberg, Laccher See district, Eifel, Germany. It occurs with tridymite, fluororichterite, hematite, rutile, aegirine-augite, kinoshitalite, and fluor-apatite in vugs in volcanic rocks, and crystallized from late-stage hydrothermal fluids associated with recent volcanism. Obertiite occurs as pale-pink elongated blades and divergent aggregates. It is brittle, H = 5, Dcalc = 3.16 g/cm3, has a colorless streak, vitreous luster, and does not fluoresce; it has perfect cleavage on {110} and conchoidal fracture. In plane-polarized light, obertiite is slightly pleochroic in shades of pink to red-orange; X ^ a = 2° (in β obtuse), Z = b, Y ^ c = 12° (in b obtuse) with absorption X ~ Y ~ Z. It is biaxial negative, α = 1.643(1), β =1.657(1), γ = 1.670(3), 2Vx = 81(1)°, no dispersion visible. Obertiite is monoclinic, space group C2/m, a = 9.776(2), b = 17.919(3), c = 5.292(1) Å, b = 104.05(2)°, V = 899.3(3) Å3, Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d(I,hkl)]: 8.414(10,110), 2.705(7,331,151), 3.390(6,131), 4.467(5,040), 3.117(5,310), 2.531(5,202), 3.255(3,240), 2.577(3,061), 2.163(3,171,261), 4.013(2,111). Analysis by a combination of electron microprobe, SIMS, and crystal-structure refinement gives SiO2 54.53, Al2O3 0.15, TiO2 7.75, Fe2O3 2.61, Mn2O3 3.27, FeO 3.36, ZnO 0.08, MgO 14.13, Li2O 0.05, CaO 0.52, Na2O 9.51, K2O 0.98, F 0.55, H2O 0.20, O ≡ F -0.23, Ni, Cr, V, Cl not detected, sum 97.46 wt%. The formula unit, calculated on the basis of 24(O,OH,F) is (K0.18Na0.84)(Na1.86Ca0.08Fe2+0.06) (Mg3.09Zn0.01Li0.03Fe3+0.29Mn3+0.37Fe2+0.41Ti4+0.86Al0.03)Si8.00O22[(OH)0.20F0.26O1.54], and is close to the ideal endmember composition Na Na2 (Mg3Fe3+Ti4+) Si8 O22 O2.

The crystal structure of obertiite was refined to an R index of 2.6% using MoKa X-ray intensity data. The M1 site is split into two subsites along the b axis, M1 and M1A; the M1 site is occupied by Mg, and M1A is occupied predominantly by Ti4+ and Mn3+; M2 is occupied by Mg, Fe2+, and Fe3+, and M3 is occupied by Mg. Local bond-valence considerations suggest that O2- at O3 is linked to Ti4+Mg or Mn3+Mn3+ at the adjacent M1 and/or M1A sites, and that OH or F at O3 is linked to MgMg at the adjacent M1 sites.

Received: 1998-12-18
Accepted: 1999-8-9
Published Online: 2015-3-25
Published in Print: 2000-1-1

© 2015 by Walter de Gruyter Berlin/Boston

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