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MALEEVITE, BaB₂Si₂O₈, AND PEKOVITE, SrB₂Si₂O₈, NEW MINERAL SPECIES FROM THE DARA-I-PIOZ ALKALINE MASSIF, NORTHERN TAJIKISTAN: DESCRIPTION AND CRYSTAL STRUCTURE

Leonid A. Pautov^1,, Atali A. Agakhanov¹, Elena Sokolova² and Frank C. Hawthorne²

¹ Fersman Mineralogical Museum, Russian Academy of Sciences, Leninskii Pr. 18/2, RU-117071 Moscow, Russia
² Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

E-mail address: pla{at}fmm.ru

Maleevite, ideally Ba B₂ Si₂ O₈, and pekovite, ideally Sr B₂ Si₂ O₈, 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/cm³. Maleevite is colorless in transmitted light, biaxial negative, with 1.649(2), ß 1.656(2), 1.656(2), 2V(obs.) = 5(3)°, 2V(calc.) = 0°. Pekovite is colorless in transmitted light, biaxial negative, with 1.597(2), ß 1.627(3), 1.632(2), 2V(obs.) = 43(3)°, 2V(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) Å, V 601.6(2) ų; pekovite: a 8.155(2), b 7.919(1), c 8.921(2) Å, V 576.1(2) ų. The strongest seven lines in the X-ray powder-diffraction patterns [d (in Å)(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: SiO₂ 34.86, B₂O₃ 19.92, BaO 43.64, PbO 0.42, sum 98.84 wt.% (maleevite can contain up to 16.08 wt.% PbO); pekovite: SiO₂ 41.56, B₂O₃ 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 O₈; pekovite: (Sr_0.97Ca_0.02) B_1.97 Si_2.02 O₈. The crystal structures of both minerals were solved by direct methods and refined to R₁ 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) Å; the T(2) site is occupied by silicon with <T(2)–O> = 1.617 (maleevite) and 1.619 (pekovite) Å. Tetrahedra form a framework with channels extending along [010]. The topology of the framework is identical to that of danburite, Ca B₂ Si₂ O₈. In the crystal structure of maleevite, Ba can be regarded as [7]- or [10]-coordinated, with <Ba–O> = 2.749 or 2.863 Å; in the crystal structure of pekovite, Sr can be regarded as [7]- or [9]-coordinated, with <Sr–O> = 2.582 or 2.693 Å. In danburite, Ca is [7]- or [9]-coordinated, <Ca–O> = 2.460 or 2.585 Å. Maleevite and pekovite are the Ba and Sr analogues of danburite.

Keywords: maleevite, pekovite, new mineral species, danburite, crystal structure, chemical analysis, Dara-i-Pioz, Tien Shan, Tajikistan.

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