Abstract

Niksergievite, [Ba1.33Ca0.67Al(CO3)(OH)4][Al2(AlSi3O10)(OH)2]•nH2O, is a new phyllosilicate closely related to the surite-ferrisurite series. It was found at the −400 m level of the Tekeli Pb-Zn mine, southeast Kazakhstan (44° N, 78° E). The mineral occurs as curved plates 3–5 mm in size forming rosette-like aggregates up to 5 cm across. Associated minerals include calcite, quartz, dolomite, celsian, sphalerite, pyrite, barite, and montmorillonite. Niksergievite is white with a light greenish tint and pearly luster on cleavage planes. The streak is white and the mineral is non-fluorescent. The Mohs hardness is 1–1½. The cleavage is perfect (mica-like) on {001}. Dm = 3.16 g/cm3 and Dx = 3.21 g/cm3. The IR spectrum shows the following peaks (* shoulder): 3665*, 3640, 3405, 1630, 1454, 1105*, 1080*, 1035, 1020*, 980*, 960*, 920*, 876, 835*, 750*, 704, 625*, 560*, 535, 474, and 417 cm−1. Optically, the mineral is colorless, non-pleochroic, biaxial (–), 2V = 0–10°, α= 1.580(2), β= 1.625(2), γ = 1.625(2), and X ~ c. The chemical composition (electron microprobe, CO2 and H2O by TGA) is K2O 0.1, CaO 5.1, BaO 27.1, MgO 0.4, FeO 0.2, Al2O3 24.8, SiO2 28.7, CO2 6.1, and H2O 8.3, with a total of 100.8 wt%. The empirical formula based on (Si + Al + Mg + Fe2+) = 7 is (Ba1.27Ca0.65K0.02)1.92(Al3.49Si3.42Mg0.07Fe0.02)7.00O10.00 (CO3)0.99(OH)6.20•0.20H2O. The simplified formula is (Ba,Ca)2(Al,Si)7O10(CO3)(OH)6nH2O and the proposed structural formula is [Ba1.33Ca0.67Al(CO3)(OH)4][Al2(AlSi3O10)(OH)2]•nH2O. The mineral is monoclinic, C2/c, C2, or Cm, a 5.176(3), b 8.989(3), c 16.166(5) Å, β 96.44(6)°, V 747.4(9) Å3, Z = 2. The strongest reflections in the X-ray powder diffraction pattern are as follows [d in Å, (I) (hkl)]: 16.1(40)(001), 4.49(90)(020), 3.68(60)(014,1̅13), 2.585(100)(130, 2̅01, 1̅31), 2.230(90)(1̅34,220), 2.069(80)(043), 1.692(60)( 3̅11, 1̅51,240). It is named in honor of Prof. Nikolai Grigorievich Sergiev (1901–1960) for his contributions to the geology of Kazakhstan.

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