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Nontronite
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Nontronite from Slovakia
General
Category Smectite - Phyllosilicate
Chemical formula (CaO0.5,Na)0.3Fe3+2(Si,Al)4O10(OH)2·nH2O
Identification
Color Yellow, olive-green, green, orange, brown
Crystal habit Earthy masses
Crystal system Monoclinic
Cleavage Perfect basal
Mohs scale hardness 1.5 to 2
Luster Earthy to dull
Streak Colorless
Specific gravity 2.3
Optical properties Biaxial (-)
Refractive index nα = 1.530 - 1.580 nβ = 1.555 - 1.612 nγ = 1.560 - 1.615
Birefringence δ = 0.030 - 0.035
References [1][2][3]

Nontronite is the iron(III) rich member of the smectite group of clay minerals. Nontronites typically have a chemical composition consisting of more than ~30% Fe2O3 and less than ~12% Al2O3 (ignited basis). Nontronite generally does not exist in economic deposits like montmorillonite, although it is not an uncommon clay mineral.[4][5] Like montmorillonite, nontronite can have variable amounts of adsorbed water associated with the interlayer surfaces and the exchange cations.

A typical structural formula for nontronite is Ca.5(Si7Al.8Fe.2)(Fe3.5Al.4Mg.1)O20(OH)4.[6] The dioctahedral sheet of nontronite is composed mainly of trivalent iron (Fe3+) cations, although some substitution by trivalent aluminium (Al3+) and divalent magnesium (Mg2+) does occur. The tetrahedral sheet is composed mainly of silicon (Si4+), but can have substantial (about 1 in 8) substitiution of either Fe3+ or Al3+, or combinations of these two cations. Thus, nontronite typically is characterised by having most (usually greater than 60%) of the layer charge located in the tetrahedral sheet. The layer charge is typically balanced by divalent calcium (Ca2+) or magnesium (Mg2+).

Nontronite forms from the weathering of biotite and basalts, precipitation of iron and silicon rich hydrothermal fluids and in deep sea hydrothermal vents.[7][8] Some evidence suggests that microorganisms may play an important role in their formation.[9]

ReferencesEditar

  1. Mineral Handbook http://rruff.geo.arizona.edu/doclib/hom/nontronite.pdf
  2. Webmineral data http://webmineral.com/data/Nontronite.shtml
  3. Mindat http://www.mindat.org/min-2924.html
  4. Eggleton, 1977 Clay minerals,12:181-194
  5. Keeling et al., 2000 Clays and Clay Minerals, 48:537-548
  6. Mountainville nontronite, Gates et al., 2002 Clays and Clay Minerals, 50:223-239
  7. Bischoff, 1972, Clays and Clay Minerals, 20:217-223
  8. Eggleton 1975 American Mineralogist, 60:1063-1068)
  9. Kohler et al., 1994 Clays and Clay Minerals, 42:680-701



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