Camaraite, Ba3NaTi4(Fe2+,Mn)8(Si2O7)4O4(OH,F)7. II. The crystal structure and crystal chemistry of a new group-II Ti-disilicate mineral

Camaraite - ideally Ba3NaTi4Fe82+(Si2O7)(4)O-4(OH)(4)F-3 - is triclinic, space group C (1) over bar, a = 10.6965(7) angstrom, b = 13.7861(9) angstrom, c = 21.478(2) angstrom, alpha = 99.345(1)degrees, beta = 92.315(2)degrees, gamma = 89.993(2)degrees, V = 3122.6(4) angstrom(3), Z = 4, D-calc. = 4.018 g cm(-3), from the Verkhnee Espe alkaline deposit, Akjailyautas Mountains, Kazakhstan, has been solved and refined to R-1 5.87% on the basis of 6682 unique reflections (F-o > 4 sigma F). The crystal structure of camaraite can be described as a combination of a TS block and an intermediate (I) block. The TS (titanium silicate) block consists of HOH sheets (H-heteropolyhedral, O-octahedral), and is characterized by a minimal cell based on translation vectors t(1) and t(2), with t(1) similar to 5.5 and t(2) similar to 7 angstrom and t(1) boolean AND t(2) close to 90 degrees. We describe the crystal structure of camaraite using a double minimal cell, with 2t(1) and 2t(2) translations. In the O sheet, there are eight [6]-coordinated M-O sites occupied mainly by Fe2+ and Mn, with minor Fe3+, Mg, Zr, Ca and Zn with < M-O-phi > = 2.185 angstrom. Eight M-O sites give, ideally Fe-8(2+) p.f.u. In the H sheet, there are four [6]-coordinated M-H sites occupied almost solely by Ti (Ti = 4 a.p.f.u.), with < M-H-phi > = 1.963 angstrom, and eight [4]-coordinated Si sites occupied solely by Si, with < Si-O > = 1.621 angstrom. The topology of the TS block is as in Group II of the Ti-disilicates (Ti = 2 a.p.f.u. per minimal cell) in the structure hierarchy of Sokolova (2006). There are six peripheral (P) sites, four [8-12]-coordinated Ba-dominant A(P) sites, giving ideally 3 Ba p.f.u., and two [10]-coordinated Na-dominant B-P sites, giving ideally 1 Na p.f.u. There are two I blocks: the I-1 block is a layer of Ba atoms (two A(P) sites); the I-2 block is a layer of Ba (two A(P) sites) and Na atoms (two B-P sites). Along c, there are two types of linkage of TS blocks: (1) TS blocks link via A(P) cations which constitute the I-1 block, and (2) TS blocks link via common vertices of M-H octahedra (as in astrophyllite-group minerals) and A(P) and B-P cations which constitute the I-2 block. Camaraite is the only mineral of Group II with two types of linkage of TS blocks and two types of I blocks in its structure. The relation of camaraite to the Group-II minerals is discussed.