Constituent
Name
Na-Montmorillonite
IMA name
Montmorillonite-Beidellite-Nontronite series
Class
silicate mineral
Compound type
phyllosilicate
Chemical composition
Formula
$(Na_{0.07}Ca_{0.26}K_{0.005})[Al_{1.615}Fe^{3+}_{0.21}Mg_{0.28}Ti_{<0.01}][(Si_{3.945}Al_{0.055}]O_{10}(OH)_2$
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)0.07-0.3 (Ca)0.26-0.15 (Al)1.67-2.5 (Fe3+)0.21-2 (Mg)0.28-0.35 (Si)3.945-4 O12 H2
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
$Na$ Sodium element 0.07 - 0.3 1.0 constituent element main
$Ca$ Calcium element 0.26 - 0.15 1.0 constituent element main
$K$ Potassium element 0.005 1.0 constituent element impurity
$Al$ Aluminium element 1.67 - 2.5 1.0 constituent element main
$Fe^{3+}$ Iron(III) cation element 0.21 - 2 1.0 constituent element main
$Mg$ Magnesium element 0.28 - 0.35 1.0 constituent element main
$Ti$ Titanium element 0.01 1.0 constituent element impurity
$Si$ Silicon element 3.945 - 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 dry matter, not just the smectite fraction. From Mermut & Cano 2001. Thermal analysis DTA: endotherms at 185°C (shoulder at 235°C), desorption of water: 755°C, dehydroxylation; shoulder at 810°C, exotherms at 980°C. TG: Loss in dehydroxylation range: 5.53% (theory: 5%)
Chemical functions
  Function Number
$(OH)^-$ 2
Chemical bonds
  Bond Number
$O^--H$ 1
Oxides
Formula $SiO_{2}$ $Na_{2}O$ $CaO$ $K_{2}O$ $Al_{2}O_{3}$ $Fe_{2}O_{3}$ $MgO$ $TiO_{2}$ $P_{2}O_{5}$
Mass fraction (%) 61.46 1.47 1.18 0.2 22.05 4.37 2.94 0.09 0.003
Oxides comments
From Mermut & Cano 2001. Loss on heating: -550°C: 0.61-1.59%; 550-1000°C: 4.47-5.15%

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)
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