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

Obertiite is a new amphibole species from Bellerberg, Laccher See district, Eifel, Germany. It occurs with tridymite, fluororichterite, hematite, ruti le, aegirine-augite, kinoshitalite, and fluor-apatite in vugs in volcanic r ocks, and crystallized from late-stage hydrothermal fluids associated with recent volcanism. Obertiite occurs as pale-pink elongated blades and diverg ent aggregates. It is brittle, H = 5, D-calc = 3.16 g/cm(3), has a colorles s streak, vitreous luster, and does not fluoresce; it has perfect cleavage on (110) and conchoidal fracture. In plane-polarized light, obertiite is sl ightly pleochroic in shades of pink to red-orange;X-boolean AND a = 2 degre es (in beta obtuse), Z = b, Y-boolean AND c = 12 degrees (in beta obtuse) w ith absorption X similar to Y similar to Z. It is biaxial negative, alpha = 1.643(1), beta = 1.657(1), gamma = 1.670(3), 2V(x) = 81(1)degrees, no disp ersion visible. Obertiite is monoclinic, space group C2/m, a = 9.776(2), b = 17.919(3), c = 5.292(1) Angstrom, beta = 104.05(2)degrees, V = 599.3(3) A ngstrom(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), 4.46 7(5,040), 3.117(5,310), 2.531(5,202), 3.255(3,240), 2.577(3,061), 2.163(3,1 71,261), 4.013(2,111). Analysis by a combination of electron microprobe. SI MS, and crystal-structure refinement gives SiO2 54.53, Al2O3 0.15, TiO2 7.7 5, 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 de tected, sum 97.46 wt%. The formula unit, calculated on the basis of 24(O,OH ,F) is (K0.18N0.84)(Na1.86Ca0.08Fe0.062+) (Mg3.09Zn0.01Li0.03Fe0.293+Mn0.37 3+Fe0.412+Ti0.864+Al0.03)Si8.00O22[(OH)(0.20)F0.26O1.54], and is close to t he ideal end-member composition Na Na-2 (Mg3Fe3+Ti4+) Si-8 O-22 O-2. The crystal structure of obertiite was refined to an R index of 2.6% using MoK alpha X-ray intensity data. The MI site is split into two subsites alon g the b axis, MI and MIA; the M1 site is occupied by Mg, and M1A is occupie d 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- a t O3 is Linked to Ti4+Mg or Mn3+Mn3+ at the adjacent M1 and/or M1A sites, a nd that OH or F at O3 is linked to MgMg at the adjacent M1 sites.