Paratobermorite, Ca4(Al0.5Si0.5)2Si4O16(OH)·2H2O·(Ca·3H2O), a new tobermorite-supergroup mineral with a novel topological type of the microporous crystal structure

Abstract

Abstract A new mineral paratobermorite with the ideal crystal-chemical formula Ca4(Al0.5Si0.5)2Si4O16 (OH)·2H2O·(Ca·3H2O) is a member of the tobermorite group within the tobermorite supergroup. It was found at the Bazhenovskoe chrysotile asbestos deposit, Central Urals, Russia, in cavities of grossular rodingite in association with prehnite, pectolite, thomsonite-Ca, and calcite. Paratobermorite occurs as well-shaped prismatic to acicular crystals up to 1 × 1.5 × 8 mm3 typically assembled in spray- or bush-like radial clusters or open-work aggregates up to 1.5 cm across, which form interrupted crusts up to 3 × 5 cm2. Paratobermorite is transparent, colorless, pale yellowish, pale beige, or pinkish, with a vitreous luster. The mineral is brittle, with the (001) perfect cleavage. The Mohs hardness is ca. 3½. Dmeas = 2.51 (2) and Dcalc = 2.533 g/cm3. Paratobermorite is optically biaxial (+), α = 1.565 (2), β = 1.566 (2), γ = 1.578 (2), 2 Vmeas = 25 (10)° and 2 Vcalc = 32° (589 nm). Optical orientation is: X = c, Y = b, Z = a. The chemical composition of paratobermorite (electron microprobe, H2O by selective sorption from gaseous products of heating) is Na2O 0.40, K2O 0.28, CaO 36.60, MnO 0.04, BaO 0.07, Al2O3 6.46, SiO2 42.32, H2O 14.10, total 100.27 wt%. The empirical formula calculated on the basis of 22 O atoms per formula unit and (O,OH)17·5H2O is Na0.09K0.04Ca4.72Al0.92Si5.09O15.69(OH)1.31·5H2O. Like other members of the tobermorite supergroup, paratobermorite displays OD character, with two MDO (maximum degree of order) structures: one (MDO1), with non-standard space group F2/d11 and the second (MDO2), just corresponding to the structure-type of the new mineral, with non-standard space group C1121/m; its unit-cell parameters obtained from single-crystal X-ray diffraction data are: a = 11.2220(4), b = 7.3777(2), c = 22.9425(8) Å, γ = 89.990(3)°, V = 1899.46(10) Å3, and Z = 4; polytype 2M. The structure of paratobermorite is solved on a single crystal, R = 8.36%. Like structures of other “tobermorites 11 Å,” it is based on the complex layer built of a sheet of sevenfold Ca-centered polyhedra with wollastonite-type chains of T tetrahedra attached to the Ca-sheet from both sides. The tetrahedral (T) sites T1 and T2 are fully occupied by Si, while alternating T3 and T4 sites are filled by Al and Si in the ratio 1:1. The chains of tetrahedra belonging to neighboring complex layers share common oxygen vertices of the bridging T3,4 tetrahedra to form xonotlite-type ribbons [Si6O17]∞. The heteropolyhedral Ca-T-O scaffolding appears as a microporous quasi-framework with wide channels, which contain additional Ca atoms and H2O molecules. The complex Ca-T-O layers in paratobermorite (so-called “complex modules of type A”) significantly differ in topology (mutual arrangement of T tetrahedra and Ca polyhedra) from the complex Ca-T-O layers in tobermorite (“complex modules of type B”). IR spectrum confirms the presence of nonequivalent H2O molecules and nonequivalent T-O-T angles involving T atoms of two neighboring wollastonite-type chains. Due to the original topological type of the structure and the presence of significant amount of Al, which substitutes Si, paratobermorite can be considered as a novel microporous material, a perspective cation-exchanger.