• journal_title：The Canadian Mineralogist
• Contributor：Elena Sokolova ; Frank C. Hawthorne ; Giancarlo Della Ventura ; Pavel M. Kartashov
• Publisher：Mineralogical Association of Canada
• Date：2004-
• Format：text/html
• Language：en
• Identifier：10.2113/gscanmin.42.4.1013
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：42
• issue：4
• firstpage：1013
• section：Articles

The crystal structure of annealed chevkinite-(Ce), ideally (REE,Ca)₄ Fe²⁺ (Ti,Fe³⁺,Nb)₄ Si₄ O₂₂, monoclinic, C2/m, crystal (2^an) a 13.400(1), b 5.7232(4), c 11.0573(9) Å, β 100.537(2)°, V 833.7(2) ų and crystal (3^an) a 13.368(2), b 5.7243(6), c 11.041(1) Å, β 100.516(1)°, V 830.7(2) ų; Z = 2, from Mongolia, has been solved by direct methods and refined to R₁ values of 6.3 and 5.7% using 1287 and 1180 unique (|F_o| > 4σF) reflections collected with a single-crystal diffractometer and MoKα radiation. Electron-microprobe analysis gave: crystal (2^an) SiO₂ 18.81, TiO₂ 15.66, FeO 11.33, MnO 0.24, CaO 1.61, Nb₂O₅ 1.55, ThO₂ 0.40, Y₂O₃ 0.30, Ce₂O₃ 23.45, La₂O₃ 13.92, Nd₂O₃ 6.88, Pr₂O₃ 1.95, Sm₂O₃ 0.51, Gd₂O₃ 0.42, HfO₂ 0.01, ZrO₂ 0.28, sum 97.32 wt.%, and crystal (3^an) SiO₂ 18.53, TiO₂ 16.11, FeO 12.62, MnO 0.31, BaO 0.02, CaO 1.21, Nb₂O₅ 1.69, ThO₂ 0.10, Y₂O₃ 0.30, Ce₂O₃ 24.30, La₂O₃ 12.94, Nd₂O₃ 8.13, Pr₂O₃ 2.44, Sm₂O₃ 0.82, Gd₂O₃ 0.36, HfO₂ 0.16, ZrO₂ 0.08, Ta₂O₅ 0.03, WO₃ 0.02, sum 100.17 wt.%. The corresponding chemical formulae are: (2^an): (Ce_1.85 La_1.11 Nd_0.53 Pr_0.15 Sm_0.04 Gd_0.03 Ca_0.32 Th_0.02)_{∑ 4.05} (Fe_0.85²⁺ Mn_0.04²⁺ Zr_0.03 Y_0.03 Ca_0.05)_{∑ 1.00} (Ti_2.54 Fe_1.19³⁺ Nb_0.15)_∑3.88 Si_4.06 O₂₂ and (3^an): (Ce_1.88 La_1.01 Nd_0.61 Pr_0.19 Sm_0.06 Gd_0.03 Ca_0.27 Y_0.03 Th_0.01)_∑4.08 (Fe_0.93²⁺ Mn_0.06 Zr_0.01)_∑1.00 (Ti_2.55 Fe_1.29³⁺ Nb_0.16)_∑4.00 Si_3.91 O₂₂ based on 13 cations per formula unit. Infrared spectra of unannealed crystals suggest significant radiation-induced damage. The structures of two unannealed crystals were refined, but there are major discrepancies (~13%) between the sums of the site-scattering values at the octahedrally coordinated M sites and the analogous values calculated from the chemical compositions determined by electron-microprobe analysis. These discrepancies were not present when data were collected on annealed crystals, indicating that the original crystals are partly metamict and that this can significantly affect the results of site-scattering refinement. In the crystal structure of chevkinite, there are two distinct (SiO₄) tetrahedra that share one vertex to form an [Si₂O₇] group: <Si–O> = 1.614 (2^an) and 1.612 (3^an) Å, <Si(1)–O(8)–Si(2)> = 172.0° (2^an) and 170.6° (3^an). There are four octahedrally coordinated M sites that are occupied by small- to medium-sized divalent to pentavalent cations. The M(1) site is occupied dominantly by Fe²⁺ with minor Mn²⁺, and the M(2), M(3) and M(4) sites are occupied by Ti, Fe³⁺ and Nb. There are two A-sites that are occupied by REE and minor Ca and Th, with <A–O> = 2.56 (2^an) and 2.55 (3^an) ([8]-coordinated) and 2.66 (2^an) and 2.55 (3^an) Å ([10]-coordinated). There are two crystallographically distinct rutile-like chains of octahedra ([M(3) + M(4)] and [M(2) + M(2)]) that extend in the b direction and link to form a layer of octahedra parallel to (001). The layers of octahedra link through [Si₂O₇] groups to form a framework with two interstitial A sites, CN = [8] and [10], in which Ce is dominant over other REE, Ca and Th.