CHEVKINITE-(Ce): CRYSTAL STRUCTURE AND THE EFFECT OF MODERATE RADIATION-INDUCED DAMAGE ON SITE-OCCUPANCY REFINEMENT

The crystal structure of annealed chevkinite-(Ce), ideally (REE,Ca)4 Fe 2+ (Ti,Fe 3+ ,Nb)4 Si4 O22, monoclinic, C2/m, crystal (2 an ) a 13.400(1), b 5.7232(4), c 11.0573(9) A, � 100.537(2), V 833.7(2) A 3 and crystal (3 an ) a 13.368(2), b 5.7243(6), c 11.041(1) A, � 100.516(1), V 830.7(2) A 3 ; 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 (|Fo| > 4� F) reflections collected with a single-crystal diffractometer and MoK� 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 ): (Ce1.85 La1.11 Nd0.53 Pr0.15 Sm0.04 Gd0.03 Ca0.32 Th0.02)� 4.05 (Fe 2+ 0.85 Mn 2+ 0.04 Zr0.03 Y0.03 Ca0.05)� 1.00 (Ti2.54 Fe 3+ 1.19 Nb0.15)� 3.88 Si4.06 O22 and (3 an ): (Ce1.88 La1.01 Nd0.61 Pr0.19 Sm0.06 Gd0.03 Ca0.27 Y0.03 Th0.01)� 4.08 (Fe 2+ 0.93 Mn0.06 Zr0.01)� 1.00 (Ti2.55 Fe 3+ 1.29 Nb0.16)� 4.00 Si3.91 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: = 1.614 (2 an ) and 1.612 (3 an ) A, = 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 Fe 2+ with minor Mn 2+ , and the M(2), M(3) and M(4) sites are occupied by Ti, Fe 3+ and Nb. There are two A-sites that are occupied by REE and minor Ca and Th, with = 2.56 (2 an ) and 2.55 (3 an ) ([8]-coordinated) and 2.66 (2 an ) and 2.55 (3 an ) A ([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.