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
Authors: Pekov I. V. , Yapaskurt V. O. , Polekhovsky Y. S. , Vigasina M. F. , Siidra O. I. 
Published: July 2014

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.