A new anhydrous amphibole from the Eifel region, Germany: Description and crystal structure of obertiite, Na[Na.sub.2] ([Mg.sub.3][Fe.sup.3+][Ti.sup.4+])[Si.sub.8][O.sub.22][O.sub.2]

Obertiite is a new amphibole species from Bellerberg, Laccher See district, Eifel, Germany. It occurs with tridymite, fluororichterite, hematite, futile, 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.sub.calc] = 3.16 g/[cm.sup.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 [degrees] (in [Beta] obtuse), Z = b, Y ^ c = 12 [degrees] (in [Beta] obtuse) with absorption X ~ Y ~ Z. It is biaxial negative, [Alpha] = 1.643(1), [Beta] =1.657(1), [Gamma] = 1.670(3), 2[V.sub.x] = 81(1) [degrees], no dispersion 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 = 899.3(3) [[Angstrom].sup.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 Si[O.sub.2] 54.53, [Al.sub.2][O.sub.3] 0.15, Ti[O.sub.2] 7.75, [Fe.sub.2][O.sub.3] 2.61, [Mn.sub.2][O.sub.3] 3.27, FeO 3.36, ZnO 0.08, MgO 14.13, [Li.sub.2]O 0.05, CaO 0.52, [Na.sub.2]O 9.51, [K.sub.2]O 0.98, F 0.55, [H.sub.2]O 0.20, O [equivalent] 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 ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and is close to the ideal end-member composition Na [Na.sub.2] ([Mg.sub.3][Fe.sup.3+][Ti.sup.4+]) [Si.sub.8] [O.sub.22] [O.sub.2]. The crystal structure of obertiite was refined to an R index of 2.6% using MoK[Alpha] 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.sup.4+] and [Mn.sup.3+]; M2 is occupied by Mg, [Fe.sup.2+], and [Fe.sup.3+], and M3 is occupied by Mg. Local bond-valence considerations suggest that [O.sup.2-] at O3 is linked to [Ti.sup.4+]Mg or [Mn.sup.3+][Mn.sup.3+] at the adjacent M1 and/or M1A sites, and that OH or F at 03 is linked to MgMg at the adjacent M1 sites.