• journal_title：The Canadian Mineralogist
• Contributor：Yulia A. Uvarova ; Elena Sokolova ; Frank C. Hawthorne ; Atali A. Agakhanov ; Leonid A. Pautov ; Vladimir Yu. Karpenko
• Publisher：Mineralogical Association of Canada
• Date：2006-
• Format：text/html
• Language：en
• Identifier：10.2113/gscanmin.44.6.1341
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：44
• issue：6
• firstpage：1341
• section：Articles

The crystal structure of senkevichite, ideally Cs K Na Ca₂ Ti⁴⁺ O [Si₇O₁₈(OH)], triclinic, P1̅, a 10.4191(4), b 12.2408(5), c 7.0569(3) Å, α 90.857(1), β 99.193(1), γ 91.895(1)°, V 887.8(1) ų, Z = 2, D_calc 3.125 g/cm³, from Dara-i-Pioz, Tien-Shan Mountains, Tajikistan, has been refined to R₁ = 4.5% for 4872 unique (F_o > 4σF) reflections collected on a Bruker single-crystal P4 diffractometer equipped with a 4K CCD detector and MoKα X-radiation. Electron-microprobe analysis gave SiO₂ 51.08, TiO₂ 8.94, FeO 0.50, MnO 2.59, CaO 10.98, K₂O 6.13, Na₂O 3.76, Nb₂O₅ 0.64, Cs₂O 15.28, (H₂O)_calc 1.09, sum 100.99 wt.%, and (H₂O) was determined from crystal-structure analysis. Senkevichite is isostructural with tinaksite and tokkoite. There are seven tetrahedrally coordinated Si sites with a grand <Si–O> distance of 1.623 Å. Six of these sites are coordinated by O atoms, and the Si(7) site is coordinated by three O atoms and one (OH) group, i.e., this is an acid silicate group. There are three [6]-coordinated M sites. The M(1) site is occupied primarily by Ti⁴⁺ (with minor Nb), with <M(1)–O> = 1.985 Å. The M(2) site is occupied solely by Ca, with <M(2)–O> = 2.382 Å, and the M(3) site is occupied by Ca (with minor Fe²⁺ and Mn²⁺), with <M(3)–O> = 2.317 Å. There is one [7]-coordinated Na site with <Na–O> = 2.504 Å. There are two A sites, the [12]-coordinated A(1) site, occupied mainly by Cs (with minor K), with <A(1)–O> = 3.318 Å, and the [10]-coordinated A(2) site, occupied solely by K, with <A(2)–O> = 2.987Å. Senkevichite is a Cs-analogue of tinaksite, K₂ Na Ca₂ Ti⁴⁺ O [Si₇O₁₈(OH)], and a Cs–Na–Ti⁴⁺ oxyanalogue of tokkoite, K₂ Ca₄ F [Si₇O₁₈(OH)].