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

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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, D calc = 3.16 g/cm 3 , 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 β obtuse) with absorption X ~ Y ~ Z . It is biaxial negative, α = 1.643(1), β =1.657(1), γ = 1.670(3), 2 V x = 81(1)°, no dispersion visible. Obertiite is monoclinic, space group C 2/ m , a = 9.776(2), b = 17.919(3), c = 5.292(1) A, β = 104.05(2)°, V = 899.3(3) A 3 , 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 SiO 2 54.53, Al 2 O 3 0.15, TiO 2 7.75, Fe 2 O 3 2.61, Mn 2 O 3 3.27, FeO 3.36, ZnO 0.08, MgO 14.13, Li 2 O 0.05, CaO 0.52, Na 2 O 9.51, K 2 O 0.98, F 0.55, H 2 O 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 (K 0.18 Na 0.84 )(Na 1.86 Ca 0.08 Fe 2+ 0.06 ) (Mg 3.09 Zn 0.01 Li 0.03 Fe 3+ 0.29 Mn 3+ 0.37 Fe 2+ 0.41 Ti 4+ 0.86 Al 0.03 )Si 8.00 O 22 [(OH) 0.20 F 0.26 O 1.54 ], and is close to the ideal end-member composition Na Na 2 (Mg 3 Fe 3+ Ti 4+ ) Si 8 O 22 O 2 . The crystal structure of obertiite was refined to an R index of 2.6% using Mo K α 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 Ti 4+ and Mn 3+ ; M2 is occupied by Mg, Fe 2+ , and Fe 3+ , and M3 is occupied by Mg. Local bond-valence considerations suggest that O 2− at O3 is linked to Ti 4+ Mg or Mn 3+ Mn 3+ at the adjacent M1 and/or M1A sites, and that OH or F at O3 is linked to MgMg at the adjacent M1 sites.