Abstract

The crystal structure of schüllerite, ideally Na2Ba2Mg2Ti2(Si2O7)2O2F2, a 5.396(1), b 7.071(1), c 10.226(2) Å, α 99.73(3), β 99.55(3), γ 90.09(3)°, V 379.1(2) Å3, Z = 1, from the Eifel volcanic region, Germany, has been refined in the space group P1 to R = 1.33% using 2247 observed (Fo > 4σF) reflections collected with a single-crystal Bruker D8 three-circle diffractometer equipped with a rotating-anode generator (MoKα radiation), multilayer optics and an APEX-II detector. The empirical formula for schüllerite was calculated on the basis of 18 (O + F) anions: (Na1.10Ca0.43Mn0.30Fe2+0.17)∑2(Ba1.57Sr0.14K0.140.15)∑2 (Mg0.79Fe2+0.71Na0.33Fe3+0.17)∑2 (Ti1.67Fe3+0.21Nb0.09Zr0.02Al0.01)∑2Si3.95O15.93F2.07, Dcalc.= 3.879 g/cm3, Z = 1, with Fe3+/ (Fe2++Fe3+) ratio determined by Mössbauer spectroscopy. Schüllerite is a Group-IV TS-block mineral: Ti + Mg = 4 apfu. The crystal structure of schüllerite is an alternation of TS (Titanium Silicate) and I (intermediate) blocks of the ideal composition [Na2Mg2Ti2(Si2O7)2O2F2]4− and [Ba2]4+, respectively. The TS block is composed of the central O (octahedral) sheet and two adjacent H (heteropolyhedral) sheets. In the O sheet, there are two brookite-like chains of MO octahedra of the following ideal compositions: [Mg2O8]12− [MO(1)] and [Na2O8]14− [MO(2)]; the ideal composition of the O sheet is [Na2Mg2O2F2]0. The H sheet is composed of the [5]-coordinated Ti-dominant MH polyhedra and Si2O7 groups; the composition of the two H sheets is [Ti2(Si2O7)2]4−. In schüllerite, the TS block has a topology characteristic of Group IV of TS-block minerals: two H sheets connect to the O sheet such that two Si2O7 groups link to the Mg-dominant octahedra of the O sheet adjacent along t1. In the O sheet, occurrence of divalent cations at the MO(1) site results in the presence of monovalent anions, F, at the XOA site. The AP site of the H sheet is occupied mainly by Ba; the AP site is shifted from the plane of the H sheet, and Ba atoms constitute the I block of the composition [Ba2]4+. Schüllerite is the only mineral of Group IV that has (1) a brookite-like [Mg2O8]12− chain of octahedra in the O sheet; (2) [5]-coordinated Ti in the H sheet; (3) Ba atoms in the I block. The ideal structural formula of schüllerite is of the form AP2MH2MO4(Si2O7)2(XOM)2(XOA)2 : Ba2Ti2Na2Mg2Ti2(Si2O7)2O2F2, Z = 1.

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