Ekplexite

Ekplexite is a unique sulfide-hydroxide niobium-rich mineral with the formula (Nb,Mo)S
2•(Mg
1-xAl
x)(OH)
2+x.[4][2] It is unique because niobium is usually found in oxide or, eventually, silicate minerals. Ekplexite is a case in which chalcophile behaviour of niobium is shown, which means niobium present in a sulfide mineral. The unique combination of elements in ekplexite has to do with its name, which comes from a Greek world on "surprise".[4][2] The other example of chalcophile behaviour of niobium is edgarite, FeNb3S6, and both minerals were found in the same environment, which is a fenitic rock of Mt. Kaskasnyunchorr, Khibiny Massif, Kola Peninsula, Russia.[5] Analysis of the same rock has revealed the presence of two analogues of ekplexite, kaskasite (molybdenum-analogue) and manganokaskasite (molybdenum- and manganese-analogue). All three minerals belong to the valleriite group, and crystallize in the trigonal system with similar possible space groups.[6][2]

Ekplexite
General
CategorySulfide mineral
Formula
(repeating unit)		(Nb,Mo)S
2•(Mg
1-xAl
x)(OH)
2+x
Crystal systemTrigonal
Crystal class3m, 32 or 3m
Space groupP321, P3m1, P3m1
Unit cella = 3.79, c = 11.3 [Å] (approximated)
Z = 2
Identification
ColorIron-black
Crystal habitmicaceous flakes (may be arranged in chaotic or radiating aggregates), in lenticular nests
Cleavage{001}, perfect
TenacityFlexible
Mohs scale hardness1
LusterMetallic
DiaphaneityOpaque
Density3.63
Pleochroismlight gray to dark gray (strong)
Common impuritiesW, V, Fe
References[1][2][3]

Notes on chemistry

Beside niobium, molybdenum, sulfur, magnesium and aluminium ekplexite contains also relatively small amounts of tungsten, vanadium and iron.[4]

Association and environment

The rock in which contains ekplexite is classified as fenite. In this rock ekplexite associates with fluorophlogopite, nepheline, orthoclase-anorthoclasee (silicates), alabandine, edgarite, pyrite, molybdenite, tungstenite (sulfides), corundum, graphite and monazite-(Ce).[4]

Crystal structure

Crystal structure of ekplexite is described as non-commensurate. It is composed of two modules:[4]

  • MeS2 sulfide module
  • brucite-like (hydroxide) module

References
  1. Pekov., I.V., Yapaskurt, V.O., Polekhovsky, Y.S., Vigasina, M.F., and Siidra, O.I., 2014. 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. Mineralogical Magazine 78(3), 663-679.
  2. "Ekplexite: Ekplexite mineral information and data". Mindat.org. Retrieved 2016-03-04.
  3. "Ekplexite - Handbook of Mineralogy" (PDF). Handbookofmineralogy.org. Retrieved 2016-03-10.
  4. Pekov., I.V., Yapaskurt, V.O., Polekhovsky, Y.S., Vigasina, M.F., and Siidra, O.I., 2014. 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. Mineralogical Magazine 78(3), 663-679.
  5. "Edgarite: Edgarite mineral information and data". Mindat.org. Retrieved 2016-03-04.
  6. Pekov., I.V., Yapaskurt, V.O., Polekhovsky, Y.S., Vigasina, M.F., and Siidra, O.I., 2014. 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. Mineralogical Magazine 78(3), 663-679
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