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
Phyllosilicates 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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