• journal_title：The Canadian Mineralogist
• Contributor：Vladimir A. Popov ; Leonid A. Pautov ; Elena Sokolova ; Frank C. Hawthorne ; Catherine McCammon ; Lyudmila F. Bazhenova
• Publisher：Mineralogical Association of Canada
• Date：2001-
• Format：text/html
• Language：en
• Identifier：10.2113/gscanmin.39.4.1095
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：39
• issue：4
• firstpage：1095
• section：Articles

Polyakovite-(Ce), ideally REE₄ Mg Cr³⁺₂ Ti₂ Si₄ O₂₂, monoclinic, a 13.398(1), b 5.6974(5), c 11.042(1) Å, β 100.539(2)°, V 828.6(2) ų, a:b:c = 2.3516:1:1.9381, space group C2/m, Z = 2, is a new mineral species from the Ilmen Mountains, southern Urals, Russia. It is of hydrothermal origin; associated minerals are calcite, dolomite, fluororichterite, phlogopite, forsterite, monazite-(Ce), clinohumite, chromite and davidite-(Ce). It occurs as irregular grains up to 3 cm in diameter and euhedral crystals elongate along [010] (up to 2 mm) and flattened on {001}. The main forms are a{100}, c{001}, t{201}, κ{2̅01}, m{110}, o{111}, n{1̅11}, p{112}, ρ{1̅12} and x{3̅02}. It is translucent in thin fragments, black with a brown streak, has a vitreous luster, and does not fluoresce under ultraviolet light. Polyakovite-(Ce) has a Mohs hardness of 5½–6, is brittle with a conchoidal fracture, and has no cleavage or parting. The measured density is 4.75(7) g/cm³, D_calc is 5.05 g/cm³. It is highly metamict. The infrared absorption spectrum has maxima at 473, 970 and 1115 cm⁻¹. In reflected light, polyakovite-(Ce) is gray, non-pleochroic, isotropic, n in the range 1.931–1.935. The strongest five reflections in the X-ray powder-diffraction pattern of annealed material [d in Å(I)(hkl)] are: 2.715(100)(004), 3.18(50)(311), 5.44(40)(002), 3.15(40)(3̅12), and 2.849(40)(020). An electron-microprobe analysis gave SiO₂ 19.08, TiO₂ 9.49, FeO 1.09, Fe₂O₃ 4.30, MnO 0.05, MgO 2.61, CaO 1.06, Cr₂O₃ 7.42, Nb₂O₅ 3.98, ThO₂ 2.79, UO₂ 0.03, Y₂O₃ 0.38, Ce₂O₃ 24.24, La₂O₃ 15.94, Nd₂O₃ 4.76, Pr₂O₃ 2.01, Sm₂O₃ 0.38, H₂O 0.14, sum 99.75 wt.%; the Fe2+/Fe3+ ratio was derived by Mössbauer spectroscopy. The corresponding chemical formula is (Ce_1.87La_1.24Nd_0.36Pr_0.16Sm_0.03Y_0.04Ca_0.24Th_0.13)_∑4.07 (Mg_0.82Fe²⁺_0.19Mn_0.01)_∑1.02 (Cr_1.24Fe³⁺_0.68)_∑1.92(Ti_1.51Nb_0.38)_∑1.89 Si_4.03 O₂₂ [based on O = 22 apfu (atoms per formula unit)]. The mineral is named for Vladislav Olegovich Polyakov (1950–1993), who contributed greatly to our knowledge of the mineralogy of the Urals.

The crystal structure of polyakovite-(Ce) (annealed single crystal) has been solved by direct methods and refined by least-squares to an R value of 4.6% using 1074 unique observed (|F_o| > 4σF) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoKα X-radiation. Polyakovite-(Ce) is a sorosilicate of Cr³⁺, Ti⁴⁺, Fe²⁺ and REE³⁺, and the first titanosilicate where Cr³⁺ is a species-forming element. In the crystal structure of polyakovite-(Ce), there are four octahedrally coordinated M sites: M(1), 0.80 Mg + 0.20 Fe²⁺, <M(1)–O> 2.108 Å; M(2), 1.52 Ti + 0.32 Nb + 0.16 □, <M(2)–O> 1.984 Å; M(4) and M(3) are occupied by 0.64 Cr³⁺ + 0.36 Fe³⁺, <M(3,4)–O> 1.995 Å. Two distinct Si sites are occupied solely by Si, and the resulting (SiO₄) tetrahedra share one vertex to form an [Si₂O₇] group: <Si–O> 1.614 Å, Si(1)–O–Si(2) 173.2(8)°. There are two types of chains of octahedra: (1) [M(3) and M(4)] octahedra, and (2) M(2) octahedra, parallel to the b axis, form together an octahedral layer parallel to (001). In a chain, every two octahedra have a common edge. M(2–4) octahedral layers, M(1) octahedra, and [Si₂O₇] groups form a three-dimensional framework. There are two A sites (CN = [8] and [10], respectively) in which Ce³⁺ is dominant over other REE³⁺, Ca and Th⁴⁺. Polyakovite-(Ce) differs from chevkinite-(Ce), (REE,Ca)₄ (Fe²⁺,Mg) Fe³⁺₂ (Ti,Nb)₂ Si₄ O₂₂, by the prevalence of Mg at the M(1) site and Cr³⁺ at the M(3) and M(4) sites.