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Ekplexite (Nb,Mo)S2·(Mg1−xAlx)(OH)2+x, kaskasite (Mo,Nb)S2·(Mg1−xAlx)(OH)2+x and manganokaskasite (Mo,Nb)S2·(Mn1−xAlx)(OH)2+x, three new valleriite-group mineral species from the Khibiny alkaline complex, Kola peninsula, Russia

Published online by Cambridge University Press:  05 July 2018

I. V. Pekov ,
V. O. Yapaskurt ,
Y. S. Polekhovsky ,
M. F. Vigasina  and
O. I. Siidra
I. V. Pekov*
Affiliation:
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia
V. O. Yapaskurt
Affiliation:
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia
Y. S. Polekhovsky
Affiliation:
Faculty of Geology, St Petersburg State University, University Embankment 7/9, 199034 St Petersburg, Russia
M. F. Vigasina
Affiliation:
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia
O. I. Siidra
Affiliation:
Faculty of Geology, St Petersburg State University, University Embankment 7/9, 199034 St Petersburg, Russia
*
E-mail: igorpekov@mail.ru
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Abstract

Three new valleriite-group minerals, ekplexite (Nb,Mo)S2·(Mg1−xAlx)(OH)2+x, kaskasite (Mo,Nb)S2·(Mg1−xAlx)(OH)2+x and manganokaskasite (Mo,Nb)S2·(Mn1−xAlx)(OH)2+x are found at Mt Kaskasnyunchorr, Khibiny alkaline complex, Kola Peninsula, Russia. They occur in fenite consisting of orthoclase−anorthoclase and nepheline with fluorophlogopite, corundum, pyrrhotite, pyrite, rutile, monazite-(Ce), graphite, edgarite, molybdenite, tungstenite, alabandite, etc. Ekplexite forms lenticular nests up to 0.2 mm × 1 mm × 1 mm consisting of near-parallel, radiating or chaotic aggregates of flakes. Kaskasite and manganokaskasite mainly occur as flakes and their near-parallel ‘stacks’ (kaskasite: up to 0.03 mm × 1 mm × 1.5 mm; manganokaskasite: up to 0.02 mm × 0.5 mm × 1 mm) epitaxially overgrow Ti-bearing pyrrhotite partially replaced by Ti-bearing pyrite. All three new minerals are opaque, ironblack, with metallic lustre. Cleavage is {001} perfect and mica-like. Flakes are very soft, flexible and inelastic. Mohs hardness is ∼1. D(calc.) = 3.63 (ekplexite), 3.83 (kaskasite) and 4.09 (manganokaskasite) g cm−3. In reflected light all these minerals are grey, without internal reflections. Anisotropism and bireflectance are very strong and pleochroism is strong. The presence of OH groups and an absence of H2O molecules are confirmed by the Raman spectroscopy data. Chemical data (wt.%, electron probe) for ekplexite, kaskasite and manganokaskasite, respectively, are: Mg 6.25, 5.94, 0.06; Al 4.31, 3.67, 3.00; Ca 0.00, 0.04, 0.00; V 0.86, 0.16, 0.15; Mn 0.00, 0.23, 11.44; Fe 0.44, 1.44, 2.06; Nb 18.17, 13.39, 14.15; Mo 15.89, 23.18, 20.08; W 8.13, 7.59, 9.12; S 27.68, 27.09, 24.84; O 16.33, 15.66, 13.36; H (calc.) 1.03, 0.99, 0.89; total 99.09, 99.08, 99.15. The empirical formulae calculated on the basis of 2 S a.p.f.u. are: ekplexite: (Nb0.45Mo0.38W0.10V0.04)S0.97S2· (Mg0.60Al0.37Fe0.02)S0.99(OH)2.36; kaskasite: (Mo0.57Nb0.34W0.10V0.01)S1.02S2· (Mg0.58Al0.32Fe0.06Mn0.01)S0.97(OH)2.32; manganokaskasite: (Mo0.54Nb0.39W0.13V0.01)S1.07S2· (Mn0.54Al0.29Fe0.10Mg0.01)S0.94(OH)2.28. All three minerals are trigonal, space groups Pm1, P3m1 or P321, one-layer polytypes (Z = 1). Their structures are non-commensurate and consist of the MeS2-type (Me = Nb, Mo, W) sulfide modules and the brucite-type hydroxide modules. Parameters of the sulfide (main) sub-lattices (a, c in Å, V in Å3) are: 3.262(2), 11.44(2), 105.4(4) (ekplexite); 3.220(2), 11.47(2), 102.8(4) (kaskasite); 3.243(3), 11.61(1), 105.8(3) (manganokaskasite). Parameters of the hydroxide sub-lattices (a, c in Å, V in Å3) are: 3.066(2), 11.52(2), 93.8(4) (ekplexite); 3.073(2), 11.50(2), 94.0(4) (kaskasite); 3.118(3), 11.62(1), 97.9(2) (manganokaskasite). Ekplexite was named from the Greek word έκπληξη meaning surprise, for its exotic combination of major chemical constituents, kaskasite after the discovery locality and manganokaskasite as a Mn analogue of kaskasite.

Keywords

ekplexitekaskasitemanganokaskasitenew mineralvalleriite grouphydroxide-sulfidehybrid mineralnon-commensurate structureniobiummolybdenumfenitealkaline rockKhibinyKola peninsula
Type
Research Article
Information
Mineralogical Magazine , Volume 78 , Issue 3 , June 2014 , pp. 663 - 679
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2014

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