Mineral phase
IMA name
Montmorillonite-Beidellite-Nontronite series
Secondary names
Dioctahedral Smectites series
Type
Class
silicate mineral
Compound type
phyllosilicate
Chemical composition
Formula
$(Na,Ca_{0.5})_{0.3}(Al,Mg,Fe)_2.(Si,Al)_4O_{10}(OH)_2 \bullet n(H_2O)$
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)0-0.3 (Ca)0-0.15 (Mg)0-0.35 (Fe3+)0-2 (Al)0.7-2.5 (Si)3.5-4 O12 H2
Isotope mixture type
terrestrial abundance
  Formula Name Family Number min-max Isotope mole fraction State Relevance
$Na$ Sodium element 0 - 0.3 constituent element main
$Ca$ Calcium element 0 - 0.15 constituent element main
$Mg$ Magnesium element 0 - 0.35 constituent element main
$Fe^{3+}$ Iron(III) cation element 0 - 2 constituent element main
$Al$ Aluminium element 0.7 - 2.5 constituent element main
$Si$ Silicon element 3.5 - 4 constituent element main
$O$ Oxygen element 12 constituent element main
$H$ Hydrogen element 2 constituent element main
Composition comments
perfect solid solution - Common impurities: Ti, K
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 (%)

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)
End-members
Phase type
crystalline
Crystal system
monoclinic
Crystal class
prismatic
Crystal class symbol
2/m (C2h)
Crystal spacegroup
C2/m
Molar mass
389.27 - 549.07 $g/mol$
Density
2.0 - 2.7 $g/cm^3$
State NTP
solid
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
Pure color
various
Diaphaneity
translucent to opaque
Luster
earthy
Links