Polyakovite-(Ce), (REE,Ca)(4) (Mg,Fe2+) (Cr3+,Fe3+)(2) (Ti,Nb)(2) S4O22, anew metamict mineral species from the Ilmen Mountains, southern Urals, Russia: Mineral description and crystal chemistry

Polyakovite-(Ce), ideally REE4 Mg Cr-2(3+) Ti-2 Si-4 O-22, monoclinic, a 13 .398(1), b 5.6974(5), c 11.042(1) Angstrom, beta 100.539(2)degrees V 828.6( 2) Angstrom (3), 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, fluororich terite, phlogopite, forsterite, monazite-(Ce), clinohumite, chromite and da vidite-(Ce). It occurs as irregular grains up to 3 cm in diameter and euhed ral crystals elongate along [010] (up to 2 mm) and flattened on {001}. The main forms are a{100}, c{001}, t{201}, kappa{(2) over bar 01},m{110}, o{111 },n{(1) over bar 11},p{112},p{(1) over bar 12} and x{(3) over bar 02}. It i s translucent in thin fragments, black with a brown streak, has a vitreous luster, and does not fluoresce under ultraviolet light. Polyakovite-(Ce) ha s a Molts hardness of 51/2-6, is brittle with a conchoidal fracture, and ha s no cleavage or parting. The measured density is 4.75(7) g/cm(3), D-calc i s 5.05 g/cm(3). It is highly metamict. The infrared absorption spectrum has maxima at 473, 970 and 1115 cm(-1). In reflected light, polyakovite-(Ce) i s gray, non-pleochroic, isotropic, n in the range 1.931-1.935. The stronges t five reflections in the X-ray powder-diffraction pattern of annealed mate rial [d in Angstrom (I)(hkl)] are: 2.715(100)(004), 3.18(50)(311), 5.44(40) (002), 3.15(40)( 12), and 2.849(40)(020). An electron-microprobe analysis g ave 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, La2 O3 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 Mossbauer spectroscopy. The corresponding c hemical formula is (Ce1.87La1.24Nd0.36Pr0.16Sm0.03Y0.04Ca0.24 Th-0.13)(Sigm a4.07) (Mg0.82Fe0.192+Mn0.01)(Sigma1.02)(Cr1.24Fe0.683+)(Sigma1.92)(Ti1.51N b0.38)(Sigma1.89)Si-4.03 O-22 [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 bee n solved by direct methods and refined by least-squares to an R value of 4. 6% using 1074 unique observed (\F-0\ > 4 sigmaF) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoK alpha 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 coo rdinated M sites: M(1), 0.80 Mg + 0.20 Fe2+, <M(1)-O > 2.108 Angstrom; M(2) , 1.52 Ti + 0.32 Nb + 0.16 square, <M(2)-O > 1.984 Angstrom; M(4) and M(3) are occupied by 0.64 Cr3+ 0.36 Fe3+,<M(3,4)-O > 1.995 Angstrom. Two distinc t Si sites are occupied solely by Si, and the resulting (SiO4) tetrahedra s hare one vertex to form an [Si2O7] group: < Si-O > 1.614 Angstrom, Si(1)-O- Si(2) 173.2(8)degrees. 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 octa hedra have a common edge. M(2-4) octahedral layers, M(l) octahedra, and [Si 2O7] groups forma three-dimensional framework. There are two A sites CN = [ 8] and [10], respectively) in which Ce3+ is dominant over other REE3+, Ca a nd Th4+. Polyakovite-(Ce) differs from chevkinite-(Ce), (REE.Ca)(4) (Fe2+,M g) Fe-2(3+) (Ti,Nb)(2) Si-4 O-22, by the prevalence of Mg at the M(1) site and Cr3+ at the M(3) and M(4) sites.