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

Abstract Cámaraite — ideally Ba 3 NaTi 4 Fe 8 2+ (Si 2 O 7 ) 4 O 4 (OH) 4 F 3 — is triclinic, space group C , a = 10.6965(7) Å, b = 13.7861(9) Å, c = 21.478(2) Å, α = 99.345(1)°, β = 92.315(2)°, γ = 89.993(2)°, V = 3122.6(4) Å 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 >4σ F ). 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 t 1 and t 2 , with t 1 ~5.5 and t 2 ~7 Å and t 1 ^ t 2 close to 90°. We describe the crystal structure of cámaraite using a double minimal cell, with 2 t 1 and 2 t 2 translations. In the O sheet, there are eight [6]-coordinated M O sites occupied mainly by Fe 2+ and Mn, with minor Fe 3+ , Mg, Zr, Ca and Zn with <M O –ϕ> = 2.185 Å. 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 –ϕ= = 1.963 Å, and eight [4]-coordinated Si sites occupied solely by Si, with <Si—0> = 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 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. 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.

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