Baotite

Abstract: Maleevite, ideally Ba B 2 Si 2 O 8 , and pekovite, ideally Sr B 2 Si 2 O 8 , are two new mineral species found in boulders in the moraine of the Dara-i-Pioz glacier, the Alai range, Tien Shan, Garmskii district, northern Tajikistan. Both minerals occur as anhedral equant crystals from 0.5 to 2 mm in diameter. Crystals of both minerals are white to transparent with a white streak and a vitreous luster. Maleevite occurs in aegirine – microcline – quartz pegmatite in syenites with arfvedsonite, polylithionite, reedmergnerite, cesium-kupletskite, hyalotekite, albite, dusmatovite, pyrochlore, tadzhikite, tienshanite, sogdianite, stillwellite-(Ce), leucosphenite, leucophanite, willemite, danburite, zektzerite, berezanskite, baotite, cappelenite-(Y) and an unknown Y–Ca silicate. Pekovite occurs in a rock consisting mainly of quartz with subordinate pectolite, aegirine, stillwellite-(Ce), polylithionite, leucosphenite and reedmergnerite. More rarely, turkestanite, galena, calcite, kapitsaite-(Y), neptunite, sugilite, baratovite, bismuth, sphalerite, fluorite, pyrochlore, fluorapatite, and zeravshanite occur in the same rock. Pekovite commonly forms intergrowths with pectolite, quartz, strontian fluorite and aegirine. Maleevite fluoresces intense blue in short-wave ultraviolet light. Maleevite and pekovite show no cleavage, have a Mohs hardness of 7, and are brittle with uneven fracture. The observed and calculated densities are as follows: maleevite, D(obs.) = 3.78(1), D(calc.) = 3.79; pekovite, D(obs.) = 3.35(2), D(calc.) = 3.36 g/cm 3 . Maleevite is colorless in transmitted light, biaxial negative, with α 1.649(2), β 1.656(2), γ 1.656(2), 2 V (obs.) = 5(3)°, 2 V (calc.) = 0°. Pekovite is colorless in transmitted light, biaxial negative, with α 1.597(2), β 1.627(3), γ 1.632(2), 2 V (obs.) = 43(3)°, 2 V (calc.) = 44°. Both maleevite and pekovite are orthorhombic, with space-group symmetry Pnma , Z = 4, and the following unit-cell dimensions: maleevite: a 8.141(2), b 8.176(2), c 9.038(2) A, V 601.6(2) A 3 ; pekovite: a 8.155(2), b 7.919(1), c 8.921(2) A, V 576.1(2) A 3 . The strongest seven lines in the X-ray powder-diffraction patterns [ d (in A)( I )( hkl )] are: maleevite: 3.62(10)(210), 2.021(7)(033), 6.07(6)(011), 3.39(6)(121), 2.83(5)(013), 2.481(4)(131), 4.86(3)(111); pekovite: 3.62(10)(210), 3.51(9)(112), 2.786(9)(103,013,122), 3.31(8)(121), 1.982(7)(232), 5.94(6)(011), 3.01(6)(202). Chemical analysis by electron microprobe gave: maleevite: SiO 2 34.86, B 2 O 3 19.92, BaO 43.64, PbO 0.42, sum 98.84 wt.% (maleevite can contain up to 16.08 wt.% PbO); pekovite: SiO 2 41.56, B 2 O 3 23.39, SrO 34.15, CaO 0.38, sum 99.48 wt.%. The resulting empirical formulae on the basis of 8 anions are as follows: maleevite: (Ba 0.99 Pb 0.01 ) B 1.99 Si 2.01 O 8 ; pekovite: (Sr 0.97 Ca 0.02 ) B 1.97 Si 2.02 O 8 . The crystal structures of both minerals were solved by direct methods and refined to R 1 indices of 2.2 (maleevite) and 3.2% (pekovite) based on 879 (maleevite) and 705 (pekovite) observed unique reflections. In the crystal structures of maleevite and pekovite, there are two tetrahedrally coordinated sites: the T (1) site is occupied by boron with T (1)–O> = 1.473 (maleevite) and 1.474 (pekovite) A; the T (2) site is occupied by silicon with T (2)–O> = 1.617 (maleevite) and 1.619 (pekovite) A. Tetrahedra form a framework with channels extending along [010]. The topology of the framework is identical to that of danburite, Ca B 2 Si 2 O 8 . In the crystal structure of maleevite, Ba can be regarded as [7]- or [10]-coordinated, with = 2.749 or 2.863 A; in the crystal structure of pekovite, Sr can be regarded as [7]- or [9]-coordinated, with = 2.582 or 2.693 A. In danburite, Ca is [7]- or [9]-coordinated, = 2.460 or 2.585 A. Maleevite and pekovite are the Ba and Sr analogues of danburite.

Abstract: As well as world class Fe and REE resources the Bayan Obo mineral deposits also hosts significant niobium resources (estimated as 2.2Mt Nb with an average grade of 0.13 weight % Nb). Niobium in this study is primarily hosted in aeschynite-(Ce) and –(Nd), but with subsidiary amounts of pyrochlore, fergusonite-(Ce), fersmite and columbite. Here we report on the paragenetic and textural setting of aeschynite, pyrochlore and fergusonite in the main ore bodies and in a carbonatite dyke. Niobium in a carbonatite sample is hosted in a phase tentatively (due to significant Ca, Mn and Ti contents) identified as fergusonite-(Ce). Aeschynite occurs overgrowing foliation in banded ores, in fractures and vugs in aegirine-rich rocks and in calcite veins. The composition in all settings is similar, but some examples in banded ores develop significant zonation in Y, Th and the REE, inferred to relate to buffering of halogen acid species to low levels by dissolution and fluoritisation of calcite, and the preferential precipitation of LREE from solution due to lower mineral solubility products compared to the HREE. Although lower in total concentration the ratios of REE in pyrochlore are similar to those of aeschynite and suggest the same metal source. The crystallisation of pyrochlore probably relates to growth in paragenetic settings where carbonates had already been eliminated and hence the buffering of F-species activities in the hydrothermal fluid was reduced. Both aeschynite and pyrochlore show evidence of alteration. Primary alteration of aeschynite resulted in leaching of A-site cations (Ca, REE, Th) and Nb, addition of Fe, and ultimately replacement by Ba-Ti phases (baotite and bafertisite). Secondary, metamictisation enhanced, possibly supergene alteration of pyrochlore resulted in hydration, leaching of A-site cations leading to the development of lattice vacancies and increases in Si. The presence of hydrothermal Nb resources at Bayan Obo suggests there may be potential for further Nb discoveries in the area, whilst the trends in element mobility during alteration have significant implications for the utility of A-B oxides as components of materials for immobilisation of radionuclides.