Original paper
Alexkhomyakovite, K6(Ca2Na)(CO3)5Cl∙6H2O, a new mineral from the Khibiny alkaline complex, Kola peninsula, Russia
Pekov, Igor V.; Zubkova, Natalia V.; Yapaskurt, Vasiliy O.; Lykova, Inna S.; Chukanov, Nikita V.; Belakovskiy, Dmitry I.; Britvin, Sergey N.; Turchkova, Anna G.; Pushcharovsky, Dmitry Y.
European Journal of Mineralogy Volume 31 Number 1 (2019), p. 135 - 143
18 references
published: Feb 21, 2019
published online: Sep 3, 2018
manuscript accepted: Jun 2, 2018
manuscript revision received: Jun 1, 2018
manuscript received: Apr 30, 2018
Abstract
The new mineral alexkhomyakovite K6(Ca2Na)(CO3)5Cl∙6H2O (IMA2015-013) occurs in a peralkaline pegmatite at Mt. Koashva, Khibiny alkaline complex, Kola peninsula, Russia. It is a hydrothermal mineral associated with villiaumite, natrite, potassic feldspar, pectolite, sodalite, biotite, lamprophyllite, titanite, fluorapatite, wadeite, burbankite, rasvumite, djerfisherite, molybdenite and an incompletely characterized Na–Ca silicate. Alexkhomyakovite occurs as equant grains up to 0.2 mm, veinlets up to 3 cm long and up to 1 mm thick and fine-grained aggregates replacing delhayelite. Alexkhomyakovite is transparent to translucent, colourless, white or grey, with vitreous to greasy lustre. It is brittle, the Mohs hardness is ca. 3. No cleavage was observed, the fracture is uneven. D meas = 2.25(1), D calc = 2.196 g cm−3. Alexkhomyakovite is optically uniaxial (–), ω = 1.543(2), ε = 1.476(2). The infrared spectrum is reported. The chemical composition [wt%, electron microprobe data, CO2 and H2O contents calculated for 5 (CO3) and 6 (H2O) per formula unit (pfu), respectively] is: Na2O 4.09, K2O 35.72, CaO 14.92, MnO 0.01, FeO 0.02, SO3 0.11, Cl 4.32, CO2 28.28, H2O 13.90, –O=Cl –0.98, total 100.39. The empirical formula calculated on the basis of 9 metal cations pfu is K5.90Ca2.07Na1.03(CO3)5(SO4)0.01O0.05Cl0.95∙6H2O. The numbers of CO3 groups and H2O molecules are based on structure data. Alexkhomyakovite is hexagonal, P63/mcm, a = 9.2691(2), c = 15.8419(4) Å, V = 1178.72(5) Å3 and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d Å(I)(hkl)] are: 7.96(27)(002), 3.486(35)(113), 3.011(100)(114), 2.977(32)(211), 2.676(36)(300), 2.626(42)(213, 115), 2.206(26)(311) and 1.982(17)(008). The crystal structure (solved from single-crystal X-ray diffraction data, R = 0.0578) is unique. It is based on (001) heteropolyhedral layers of pentagonal bipyramids (Ca,Na)O5(H2O)2 interconnected via carbonate groups of two types, edge-sharing ones and vertex-sharing ones. Ca and Na are disordered. Ten-fold coordinated K cations centre KO6Cl(H2O)3 polyhedra on either side of the heteropolyhedral layer. A third type of carbonate group and Cl occupy the interlayer. The mineral is named in honour of the outstanding Russian mineralogist Alexander Petrovich Khomyakov (1933–2012).
Keywords
alexkhomyakovite • new mineral • potassium carbonate hydrate • crystal structure • peralkaline pegmatite • Koashva apatite deposit • Khibiny alkaline complex • Kola peninsula