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

Abstract

The crystal structure of schullerite, ideally Na 2 Ba 2 Mg 2 Ti 2 (Si 2 O 7 ) 2 O 2 F 2 , a 5.396(1), b 7.071(1), c 10.226(2) A, α 99.73(3), β 99.55(3), γ 90.09(3)°, V 379.1(2) A 3 , Z = 1, from the Eifel volcanic region, Germany, has been refined in the space group P 1 to R = 1.33% using 2247 observed ( F o > 4σ F ) reflections collected with a single-crystal Bruker D8 three-circle diffractometer equipped with a rotating-anode generator (Mo K α radiation), multilayer optics and an APEX-II detector. The empirical formula for schullerite was calculated on the basis of 18 (O + F) anions: (Na 1.10 Ca 0.43 Mn 0.30 Fe 2+ 0.17 ) ∑2 (Ba 1.57 Sr 0.14 K 0.14 □ 0.15 ) ∑2 (Mg 0.79 Fe 2+ 0.71 Na 0.33 Fe 3+ 0.17 ) ∑2 (Ti 1.67 Fe 3+ 0.21 Nb 0.09 Zr 0.02 Al 0.01 ) ∑2 Si 3.95 O 15.93 F 2.07 , D calc. = 3.879 g/cm 3 , Z = 1, with Fe 3+ / (Fe 2+ +Fe 3+ ) ratio determined by Mossbauer spectroscopy. Schullerite is a Group-IV TS-block mineral: Ti + Mg = 4 apfu . The crystal structure of schullerite is an alternation of TS (Titanium Silicate) and I (intermediate) blocks of the ideal composition [Na 2 Mg 2 Ti 2 (Si 2 O 7 ) 2 O 2 F 2 ] 4− and [Ba 2 ] 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: [Mg 2 O 8 ] 12− [M O (1)] and [Na 2 O 8 ] 14− [M O (2)]; the ideal composition of the O sheet is [Na 2 Mg 2 O 2 F 2 ] 0. The H sheet is composed of the [5]-coordinated Ti-dominant M H polyhedra and Si 2 O 7 groups; the composition of the two H sheets is [Ti 2 (Si 2 O 7 ) 2 ] 4−. In schullerite, 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 Si 2 O 7 groups link to the Mg-dominant octahedra of the O sheet adjacent along t 1 . 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 [Ba 2 ] 4+ . Schullerite is the only mineral of Group IV that has (1) a brookite-like [Mg 2 O 8 ] 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 schullerite is of the form A P 2 M H 2 M O 4 (Si 2 O 7 ) 2 (X O M ) 2 (X O A ) 2 : Ba 2 Ti 2 Na 2 Mg 2 Ti 2 (Si 2 O 7 ) 2 O 2 F 2 , Z = 1.