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 Cr 3+ 2 Ti 2 Si 4 O 22 , monoclinic, a 13.398(1), b 5.6974(5), c 11.042(1) Å, ␤ 100.539(2)°, V 828.6(2) Å 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, 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}, {201}, m{110}, o{111}, n{111}, p{112}, {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/cm 3 , D calc is 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) 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., sum 99.75 wt.%; the Fe 2+ /Fe 3+ ratio was derived by Mössbauer spectroscopy. The corresponding chemical formula is (Ce [based on O = 22 apfu (atoms per formula unit)]. The mineral is named for Vladislav Olegovich Polyakov , 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 leastsquares to an R value of 4.6% using 1074 unique observed (|F o | > 4F) reflections collected with a single-crystal diffractometer fitted with a CCD detector and MoK␣ X-radiation. Polyakovite-(Ce) is a sorosilicate of Cr 3+ , Ti 4+ , Fe 2+ and REE 3+ , and the first titanosilicate where Cr 3+ 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 Fe 2+ , <M(1)-O> 2.108 Å; M(2), 1.52 Ti + 0.32 Nb + 0.16 Ⅺ, <M(2)-O> 1.984 Å; M(4) and M(3) are occupied by 0.64 Cr 3+ + 0.36 Fe 3+ , <M(3,4)-O> 1.995 Å. Two distinct Si sites are occupied solely by Si, and the resulting (SiO 4 ) tetrahedra share one vertex to form an [Si 2 O 7 ] 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)

The chevkinite group: underestimated accessory phases from a wide range of parageneses

“…Strontiochevkinite is known from two localities, in rheomorphic fenites (sanidine-aegirine-nepheline dykes) associated with carbonatite plugs at Sarambi, Paraguay (Haggerty, Mariano 1983) and in nepheline syenitic pegmatites at Pegmatite Peak, Bearpaw Mountains, Montana (Chakhmouradian, Mitchell 1999). Polyakovite-(Ce) has been recorded from two very different parageneses: from a carbonatite vein of hydrothermal origin at the contact between fenites and metasomatised ultramafic rocks in the Ilmen Mountains, southern Urals, Russia (Popov et al 2001) and as radioactive inclusions in diamonds from the River Ranch kimberlite pipe, Zimbabwe (Kopylova et al 1997). Clearly, polyakovite-(Ce) has a very wide stability field.…”

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

“…Ti is dominant at the M(2-4) sites, producing a layer of corner-and edge- sharing (TiO 6 ) octahedra. On the other hand, polyakovite-(Ce) has a different arrangement of (TiO 6 ) octahedra: Ti 4+ and (Cr 3+ , Fe 3+ ) occur at the octahedrally coordinated M(2-4) sites (Popov et al 2001) (Fig. 2c).…”

“…Usually, chevkinite-(Ce) is highly metamict, accounting for the problem in adequately characterizing its crystal structure. In addition to chevkinite-(Ce), there are two more minerals of the chevkinite-(Ce) structure-type: strontiochevkinite, ideally (Sr,REE) 4 Fe 2+ (Ti 4+ ,Fe) 4 Si 4 O 22 (Haggerty & Mariano 1983) and polyakovite-(Ce), ideally Ce 4 Mg Cr 3+ 2 Ti 4+ 2 Si 4 O 22 (Popov et al 2001), and two more minerals of the perrierite-(Ce) structure-type, rengeite, ideally Sr 4 Zr Ti 4 Si 4 O 22 (Miyajima et al 2001) and matsubaraite, Sr 4 Ti 5 Si 4 O 22 (Miyajima et al 2002). The crystal structures of strontiochevkinite and rengeite are not yet fully characterized.…”

Structure of Sr-Zr-bearing perrierite-(Ce) from the Burpala Massif, Russia

“…Plot of (CaO+SrO) vs. FeO* (wt.%) to show the relationships between type members of the chevkinite group and Sr-Zr perrierite-(Ce). FeO*, all Fe as Fe 2+ ; C, chevkinite-(Ce) (Macdonald and Belkin, 2002); D, dingdaohengite-(Ce) (Xu et al, 2008); H, hezuolinite (Yang et al, 2012); M, matsubaraite (Miyajima et al, 2002); Mp, maoniupingite-(Ce) (Shen et al, 2005); P-C, perrierite-(Ce) (Macdonald et al, 2009); P-L, perrierite-(La) (Chukanov et al, 2012b); Po, polyakovite-(Ce) (Popov et al, 2001); R, rengeite (Miyajima et al, 2001); SC, strontio-chevkinite (Haggerty and Mariano, 1983); SZ-P, Sr-Zr-bearing perrierite-(Ce), this paper.…”