Constituent
Name
Na-Montmorillonite
IMA name
Montmorillonite-Beidellite-Nontronite series
Class
silicate mineral
Compound type
phyllosilicate
Chemical composition
Formula
$({Na_{0.16}Ca_{0.06}K_{0.025})[Al_{1.505}Fe^{3+}_{0.205}Mn_{0.005}Mg_{0.27}Ti_{0.01}][(Si_{3.99}Al_{0.01}]O_{10}(OH)_2$
Chemical formula
[(Na)1-x(Ca)0.5(1+x)]0.3[(Al)x(Mg)y(Fe3+)1-x-y]2[(Si)x(Al)1-x]4O10(OH)2•n(H2O)
Elemental formula
(Na)0.16-0.3 (Ca)0.06-0.15 (Al)1.515-2.5 (Fe3+)0.205-2 (Mg)0.27-0.35 (Si)3.99-4 O12 H2
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
$Na$ Sodium element 0.16 - 0.3 1.0 constituent element main
$Ca$ Calcium element 0.06 - 0.15 1.0 constituent element main
$K$ Potassium element 0.025 1.0 constituent element impurity
$Al$ Aluminium element 1.515 - 2.5 1.0 constituent element main
$Fe^{3+}$ Iron(III) cation element 0.205 - 2 1.0 constituent element main
$Mn$ Manganese element 0.005 1.0 constituent element impurity
$Mg$ Magnesium element 0.27 - 0.35 1.0 constituent element main
$Ti$ Titanium element 0.01 1.0 constituent element impurity
$Si$ Silicon element 3.99 - 4 1.0 constituent element main
$O$ Oxygen element 12 1.0 constituent element main
$H$ Hydrogen element 2 1.0 constituent element main
Composition comments
perfect solid solution - Common impurities: Ti, K - This composition is for the whole matter, mot just the smectite fraction. Thermal analysis DTA: endotherms at 185oC (shoulder at 235oC), desorption of water: 755oC, dehydroxylation; shoulder at 810oC, exotherms at 980oC. TG: Loss in dehydroxylation range: 5.53% (theory: 5%)
Chemical functions
  Function Number
$(OH)^-$ 2
Chemical bonds
  Bond Number
$O^--H$ 1
Oxides
Formula $Na_{2}O$ $CaO$ $Al_{2}O_{3}$ $MgO$ $Fe_{2}O_{3}$ $SiO_{2}$ $H_{2}O$
Mass fraction (%)
Oxides comments
not measured only on smectite 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)
Phase type
crystalline
Crystal system
monoclinic
Crystal class
prismatic
Crystal class symbol
2/m (C2h)
Crystal spacegroup
C2/m
Texture
cryptocrystalline
Molar mass
389.27 - 549.07 $g/mol$
Density
2.0 - 2.7 $g/cm^3$
Refringence type
biaxial
Refringence sign
negative
Birefringence
0.015 - 0.035
Refraction index $n_{\alpha}$
1.485 - 1.58
Refraction index $n_{\beta}$
1.504 - 1.612
Refraction index $n_{\gamma}$
1.505 - 1.615
Color
gray-green
Diaphaneity
translucent to opaque
Luster
earthy