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Niksergievite, [Ba 1.33 Ca 0.67 Al(CO 3 )(OH) 4 ][Al 2 (AlSi 3 O 10 )(OH) 2 ]·nH 2 O, 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 1.11/2. The cleavage is perfect (mica-like) on {001}. D m  = 3.16 g/cm 3  and D x  = 3.21 g/cm 3 . 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, CO 2  and H 2 O by TGA) is K 2 O 0.1, CaO 5.1, BaO 27.1, MgO 0.4, FeO 0.2, Al 2 O 3  24.8, SiO 2  28.7, CO 2  6.1, and H 2 O 8.3, with a total of 100.8 wt%. The empirical formula based on (Si + Al + Mg + Fe) = 7 is (Ba 1.27 Ca 0.65 K 0.02 ) 1.92 (Al 3.49 Si 3.42 Mg 0.07 Fe 2+  0.02 ) 7.00 O 10.00 (CO 3 ) 0.99 (OH) 6.20 · 0.20 H 2 O. The simplified formula is (Ba,Ca) 2 (Al,Si) 7 O 10 (CO 3 )(OH) 6 ·nH 2 O and the proposed structural formula is [Ba 1.33 Ca 0.67 Al(CO 3 )(OH) 4 ][Al 2 (AlSi 3 O 10 )(OH) 2 ]·nH 2 O. 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) Å 3 , 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)(014,1̄13), 2.585(100)(130,2̅01, 1̄31), 2.230(90)(1.34,220), 2.069(80)(043), 1.692(60)(3.11,1̄51,240). It is named in honor of Prof. Nikolai Grigorievich Sergiev (1901.1960) for his contributions to the geology of Kazakhstan.

Niksergievite, [Ba 1.33 Ca 0.67 Al(CO 3 )(OH) 4 ][Al 2 (AlSi 3 O 10 )(OH)2]·nH 2 O, a new phyllosilicate related to the surite-ferrisurite series