POLYAKOVITE-(Ce), (REE,Ca)4 (Mg,Fe2+) (Cr3+,Fe3+)2 (Ti,Nb)2 Si4 O22, A NEW METAMICT MINERAL SPECIES FROM THE ILMEN MOUNTAINS, SOUTHERN URALS, RUSSIA: MINERAL DESCRIPTION AND CRYSTAL CHEMISTRY

Abstract

Polyakovite-(Ce), ideally REE 4 Mg Cr3+2 Ti2 Si4 O22, monoclinic, a 13.398(1), b 5.6974(5), c 11.042(1) A, β 100.539(2)°, V 828.6(2) A3, a:b:c = 2.3516:1:1.9381, space group C 2/ 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{201}, m {110}, o {111}, n {111}, p {112}, ρ{112} and x {302}. 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 A(I)( hkl )] are: 2.715(100)(004), 3.18(50)(311), 5.44(40)(002), 3.15(40)(312), 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 Mossbauer spectroscopy. The corresponding chemical formula is (Ce1.87La1.24Nd0.36Pr0.16Sm0.03Y0.04Ca0.24Th0.13)∑4.07 (Mg0.82Fe2+0.19Mn0.01)∑1.02 (Cr1.24Fe3+0.68)∑1.92(Ti1.51Nb0.38)∑1.89 Si4.03 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| > 4σ F ) reflections collected with a single-crystal diffractometer fitted with a CCD detector and Mo K α X-radiation. Polyakovite-(Ce) is a sorosilicate of Cr3+, Ti4+, Fe2+ and REE 3+, 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+, 2.108 A; M (2), 1.52 Ti + 0.32 Nb + 0.16 □, 1.984 A; M (4) and M (3) are occupied by 0.64 Cr3+ + 0.36 Fe3+, 1.995 A. Two distinct Si sites are occupied solely by Si, and the resulting (SiO4) tetrahedra share one vertex to form an [Si2O7] group: 1.614 A, 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 REE 3+, 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.