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

摘要

Cámaraite ideally Ba₃NaTi₄(Si₂O₇)₄O₄(OH)₄F₃ - is triclinic, space group C, a = 10.6965(7) Å, b = 13.7861(9) Å, c = 21.478(2) Å, = 99.345(1)°, β = 92.315(2)°, = 89.993(2)°, V = 3122.6(4) Å3, Z = 4, D_calc. = 4.018 g cm-3, from the Verkhnee Espe alkaline deposit, Akjailyautas Mountains, Kazakhstan, has been solved and refined to R₁ 5.87% on the basis of 6682 unique reflections (F_o >4F). The crystal structure of cámaraite 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 t₁ and t₂, with t₁ 5.5 and t₂ 7 Å and t₁ ^ t₂ close to 90°. We describe the crystal structure of cámaraite using a double minimal cell, with 2t₁ and 2t₂ translations. In the O sheet, there are eight [6]-coordinated MO sites occupied mainly by Fe2+ and Mn, with minor Fe3+, Mg, Zr, Ca and Zn with = 2.185 Å. Eight MO sites give, ideally p.f.u. In the H sheet, there are four [6]-coordinated MH sites occupied almost solely by Ti (Ti = 4 a.p.f.u.), with = 1.963 Å, and eight [4]-coordinated Si sites occupied solely by Si, with = 1.621 Å. 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 AP sites, giving ideally 3 Ba p.f.u., and two [10]-coordinated Na-dominant BP sites, giving ideally 1 Na p.f.u. There are two I blocks: the I₁ block is a layer of Ba atoms (two AP sites); the I₂ block is a layer of Ba (two AP sites) and Na atoms (two BP sites). Along c, there are two types of linkage of TS blocks: (1) TS blocks link via AP cations which constitute the I₁ block, and (2) TS blocks link via common vertices of MH octahedra (as in astrophyllite-group minerals) and AP and BP cations which constitute the I₂ block. Cámaraite 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 cámaraite to the Group-II minerals is discussed.