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Niksergievite, [Ba_1.33Ca_0.67Al(CO₃)(OH)₄][Al₂(AlSi₃O₁₀)(OH)₂]·nH₂O, a new phyllosilicate related to the surite-ferrisurite series

¹ Satpaev Institute of Geological Sciences, Kabanbai batyr, 69a, Almaty, 480100, Kazakhstan
² Department of Mineral Deposits, St. Petersburg State University, Universitetskaya Naberezhnaya 7/9, RU-199034 St. Petersburg, Russia
³ Institute of Chemical Physics, Russian Academy of Sciences, Chernogolovka, RU-142432 Moscow oblast, Russia
⁴ Ioffe Physico-Technical Institute, Russian Academy of Sciences, Polytekhnicheskaya Ulitsa 26, RU-194021 St. Petersburg, Russia

Correspondence: ^* E-mail: sergey_britvin{at}mail.ru

Niksergievite, [Ba_1.33Ca_0.67Al(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 1–1. The cleavage is perfect (mica-like) on {001}. D_m = 3.16 g/cm³ and D_x = 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°, = 1.580(2), ß= 1.625(2), = 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 (Ba_1.27Ca_0.65K_0.02)_1.92(Al_3.49Si_3.42Mg_0.07Fe_0.02)_7.00O_10.00 (CO₃)_0.99(OH)_6.20•0.20H₂O. The simplified formula is (Ba,Ca)₂(Al,Si)₇O₁₀(CO₃)(OH)₆•nH₂O and the proposed structural formula is [Ba_1.33Ca_0.67Al(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, ß 96.44(6)°, V 747.4(9) ų, 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)(014,13), 2.585(100)(130, 01, 31), 2.230(90)(34,220), 2.069(80)(043), 1.692(60)( 11, 51,240). It is named in honor of Prof. Nikolai Grigorievich Sergiev (1901–1960) for his contributions to the geology of Kazakhstan.