Ekplexite (Nb,Mo)S₂·(Mg_1−xAl_x)(OH)_2+x, kaskasite (Mo,Nb)S₂·(Mg_1−xAl_x)(OH)_2+x and manganokaskasite (Mo,Nb)S₂·(Mn_1−xAl_x)(OH)_2+x, three new valleriite-group mineral species from the Khibiny alkaline complex, Kola peninsula, Russia

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.

Synthesis and characterization of nanoscale composite particles formed by 2D layers of Cu-Fe sulfide and Mg-based hydroxide

“…The precursors spontaneously self-assemble into alternating quasi-atomic Cu-Fe-S sulfide and Mg hydroxide sheets in the hydrothermal process through a mechanism that still needs to be understood. The structure of valleriites formed by incommensurate 2D layers allows to vary widely, and to some extent independently, their composition and then properties, including replacement both of metals and chalcogenide atoms [30][31][32][33][34][42][43][44][45][46][47][48][49][50]. On the other hand, the counterparts affect each other via the electric charges and probably other factors; this makes possible to finely adjust the material characteristics.…”

“…35[Mg0.74Al0.26(OH)2]. Minerals of the valleriite group [31][32][33][34] with iron dominance in hydroxide layers (mineral ferrovalleriite) [31,32] or chromium instead of iron in hydroxide and sulfide sheets [33], or FeS layers in place of CuFeS2 (tochilinite) [24,27,35,36] were discovered too. Valleriite is known [20,24,25,30] to crystallize in a hexagonal lattice with the sulfide sublattice being rhombohedral (space group R3m with hexagonal axes a = 0.3792 nm and c = 0.341 nm) and the hydroxide one having hexagonal lattice (space group P3m1, a = 0.307 nm and c = 1.137 nm).…”

Synthesis and characterization of nanoscale composite particles formed by 2D layers of Cu-Fe sulfide and Mg-based hydroxide

“…The flake-like nanoparticles composed of altering quasi-atomic Cu-Fe-S sulfide and Mg hydroxide sheets are manufactured in the environment-friendly hydrothermal process using common reagents. Such a chalcogenide-hydroxide incommensurate structures looks unusual and complex but there exist a number of natural [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and synthetic [43][44][45][46][47][48][49][50][51] layered materials, suggesting that those are quite stable and spontaneously assemble via a mechanism that needs to be understood. The composition and then morphology and properties of the materials can be engineered by varying the nature and concentrations of precursors, modifiers, dopants far beyond those used in this case study.…”

“…For instance, Hughes et al [30] have reported the composition of synthetic valleriites ranged from [CuFeS2]1.67[Mg0.70Al0.30(OH)2] to [Cu1.30Fe0.70S2]1.35[Mg0.74Al0.26(OH)2]. Minerals of the valleriite group [31][32][33][34] can have prevalent iron in hydroxide layers (ferrovalleriite), [31,32] or chromium instead of iron in hydroxide and sulfide sheets. [33] These findings imply that such synthetic systems should be stable and their chemical composition could be widely varied and adjusted on demand.…”

Occurrence of Tungstenite-2H in magnesian skarns of gem quality spinel deposit Kukhi-Lal (South-Western Pamir, Tajikistan)

“…, содержащие до 10 мас.% примесного вольфрама, который замещает молибден и ниобий в сульфидном модуле (Pekov et al, 2014). И.В.…”