Cámaraite ideally Ba<sub>3sub>NaTi<sub>4sub><span class="inline-formula" id="inline-formula-1">![]()
ss="math tex" alt="Formula" src="855/embed/tex-math-1.gif" />span>(Si<sub>2sub>O<sub>7sub>)<sub>4sub>O<sub>4sub>(OH)<sub>4sub>F<sub>3sub> - is triclinic, space group C<span class="inline-formula" id="inline-formula-2">![]()
ss="math tex" alt="Formula" src="855/embed/tex-math-2.gif" />span>, 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<sup>3sup>, Z = 4, D<sub>calc.sub> = 4.018 g cm<sup>-3sup>, from the Verkhnee Espe alkaline deposit, Akjailyautas Mountains, Kazakhstan, has been solved and refined to R<sub>1sub> 5.87% on the basis of 6682 unique reflections (F<sub>osub> >4σF). The crystal structure of cámaraite can be described as a combination of a TS block and an intermediate (<strong>I)strong> 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 <strong>tstrong><sub><strong>1strong>sub> and <strong>tstrong><sub><strong>2strong>sub>, with t<sub>1sub> ∼5.5 and t<sub>2sub> ∼7 Å and <strong>tstrong><sub><strong>1strong>sub> ^ <strong>tstrong><sub><strong>2strong>sub> close to 90°. We describe the crystal structure of cámaraite using a double minimal cell, with 2t<sub><strong>1strong>sub> and 2t<sub><strong>2strong>sub> translations. In the O sheet, there are eight [6]-coordinated M<sup>Osup> sites occupied mainly by Fe<sup>2+sup> and Mn, with minor Fe<sup>3+sup>, Mg, Zr, Ca and Zn with <M<sup>Osup>-φ> = 2.185 Å. Eight M<sup>Osup> sites give, ideally <span class="inline-formula" id="inline-formula-3">![]()
ss="math tex" alt="Formula" src="855/embed/tex-math-3.gif" />span> p.f.u. In the H sheet, there are four [6]-coordinated M<sup>Hsup> sites occupied almost solely by Ti (Ti = 4 a.p.f.u.), with <M<sup>Hsup>-φ> = 1.963 Å, and eight [4]-coordinated Si sites occupied solely by Si, with <Si-O> = 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 (<span id="xref-ref-13-1" class="xref-bibr">2006span>). There are six peripheral (P) sites, four [8-12]-coordinated Ba-dominant A<sup>Psup> sites, giving ideally 3 Ba p.f.u., and two [10]-coordinated Na-dominant B<sup>Psup> sites, giving ideally 1 Na p.f.u. There are two <strong>Istrong> blocks: the <strong>Istrong><sub>1sub> block is a layer of Ba atoms (two A<sup>Psup> sites); the <strong>Istrong><sub>2sub> block is a layer of Ba (two A<sup>Psup> sites) and Na atoms (two B<sup>Psup> sites). Along <strong>cstrong>, there are two types of linkage of TS blocks: (1) TS blocks link via A<sup>Psup> cations which constitute the <strong>Istrong><sub>1sub> block, and (2) TS blocks link via common vertices of M<sup>Hsup> octahedra (as in astrophyllite-group minerals) and A<sup>Psup> and B<sup>Psup> cations which constitute the <strong>Istrong><sub>2sub> block. Cámaraite is the only mineral of Group II with two types of linkage of TS blocks and two types of <strong>Istrong> blocks in its structure. The relation of cámaraite to the Group-II minerals is discussed.
