FROM STRUCTURE TOPOLOGY TO CHEMICAL COMPOSITION. XV. TITANIUM SILICATES: REVISION OF THE CRYSTAL STRUCTURE AND CHEMICAL FORMULA OF SCHÜLLERITE, Na₂Ba₂Mg₂Ti₂(Si₂O₇)₂O₂F₂, FROM THE EIFEL VOLCANIC REGION, GERMANY

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) ų, Z = 1, from the Eifel volcanic region, Germany, has been refined in the space group P1 to R = 1.33% using 2247 observed (F_o > 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: (Na_1.10Ca_0.43Mn_0.30Fe²⁺_0.17)_∑2(Ba_1.57Sr_0.14K_0.14□_0.15)_∑2 (Mg_0.79Fe²⁺_0.71Na_0.33Fe³⁺_0.17)_∑2 (Ti_1.67Fe³⁺_0.21Nb_0.09Zr_0.02Al_0.01)_∑2Si_3.95O_15.93F_2.07, D_calc.= 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 M^O octahedra of the following ideal compositions: [Mg₂O₈]¹²⁻ [M^O(1)] and [Na₂O₈]¹⁴⁻ [M^O(2)]; the ideal composition of the O sheet is [Na₂Mg₂O₂F₂]^0. The H sheet is composed of the [5]-coordinated Ti-dominant M^H 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 M^O(1) site results in the presence of monovalent anions, F⁻, at the X^O_A site. The A^P site of the H sheet is occupied mainly by Ba; the A^P 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 A^P₂M^H₂M^O₄(Si₂O₇)₂(X^O_M)₂(X^O_A)₂ : Ba₂Ti₂Na₂Mg₂Ti₂(Si₂O₇)₂O₂F₂, Z = 1.