Mineral phase
IMA name
Nontronite
Type
Class
silicate mineral
Compound type
phyllosilicate
Comments
Nontronites typically have a chemical composition consisting of more than ~30% $Fe_2O_3$, less than ~12% $Al_2O_3$ (ignited basis), and a hydration number of 4
Chemical composition
Formula
$Na_{0.3}Fe^{3+}_2.(Si,Al)_4O_{10}(OH)_2 \bullet n(H_2O)$
Chemical formula
Na0.3(Fe3+)2(Si,Al)4O10(OH)2•n(H2O)
Elemental formula
Na0.3 (Fe3+)2 (Si)3.65-4 (Al)0-0.35 O12 H2
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
$Na$ Sodium element 0.3 constituent element main
$Fe^{3+}$ Iron(III) cation element 2 constituent element main
$Si$ Silicon element 3.65 - 4 constituent element main
$Al$ Aluminium element 0 - 0.35 constituent element main
$O$ Oxygen element 12 constituent element main
$H$ Hydrogen element 2 constituent element main
Composition comments
Typical formula: $Na_{0.3}Fe^{3+}_2.(Si_3Al)O_{10}(OH)_2 \bullet 4(H_2O)$ - Common impurities: Ti, Mg, Ca
Chemical functions
  Function Number
$(OH)^-$ 2
Chemical bonds
  Bond Number
$O^--H$ 1
Oxides
Formula $Na_{2}O$ $Fe_{2}O_{3}$ $Al_{2}O_{3}$ $SiO_{2}$ $H_{2}O$
Mass fraction (%)

Strunz

Class
silicates (09)
Division
phyllosilicate (09.E)
Family
with mica sheets, composed of tetrahedral and octahedral nets (09.EC)
Code
09.EC.40

Dana

Major class
silicates (VIII)
Class
phyllosilicate sheets of six-membered rings (71)
Type
with 2:1 clays (71.03)
Group
Smectite group (Dioctahedral Smectites) (71.03.01a)
Code
71.03.01a.03
Phase type
crystalline
Crystal system
monoclinic
Crystal class
prismatic
Crystal class symbol
2/m (C2h)
Crystal spacegroup
C2/m
Molar mass
495.90 $g/mol$
Density
2.3 $g/cm^3$
State NTP
solid
Refringence type
biaxial
Refringence sign
negative
Birefringence
0.030 - 0.035
Refraction index $n_{\alpha}$
1.53 - 1.58
Refraction index $n_{\beta}$
1.555 - 1.612
Refraction index $n_{\gamma}$
1.56 - 1.615
Dispersion V
strong
Pure color
green
Color
greenish yellow, brownish green, olive green
Diaphaneity
translucent to opaque
Luster
earthy
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