3388 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
to varying extents and can be exchanged with cations from
an external solution. The exchangeable cations contained
in smectites are most commonly Na+, Ca2+ and Mg2+
ions, but can include a wide range of other cations such as
Li+, K+, H+ and organic ions. The chemical composition
of smectites is therefore varied and complex due to vari-
ous possible cationic substitutions in both the tetrahedral
and octahedral sheets (Garcia-Romero et al. 2021 Al-Ani
and Sarapää 2008 Brindley and Brown 1984). Due to
the structure, presence of exchangeable cations, and small
crystal size (2 µm) of the smectite minerals, a number of
unique properties are observed. These properties include a
high cation exchange capacity, large swelling ability in the
presence of water, as well as the ability to strongly influence
the rheology of a liquid (Odom 1984).
The end-member compositions for the most com-
mon smectites are given in Table 1, where M+ represents
the exchangeable cations in the interlayer space. Although
these exchangeable cations are represented as monovalent
ions, equivalent numbers of ions of different valencies can
be substituted. The chemical formula for the smectites is
defined in terms of half a unit cell (i.e., ten oxygen atoms
and two hydroxyl ions). In half a unit cell, there are a total
of four tetrahedral sites and three octahedral sites. In diocta-
hedral smectites, such as montmorillonite, two of the three
octahedral sites are occupied in trioctahedral smectites, all
the octahedral sites are occupied. Intermediate chemical
compositions between the end-members in the dioctahe-
dral group, and between end-members in the trioctahedral
Silica tetrahedral sheet
Silica tetrahedral sheet
Alumina octahedral sheet
Interlayer space
O
H
Si ,Al
Al ,Mg
Na ,Ca ,Mg ,Li ,K ,H
Figure 1. Simplified structure of montmorillonite, adapted from Graham (2019)
Table 1. End-member compositions for the most common smectites, where M+ represents exchangeable
cations including Na+, Ca2+, Mg2+, Li+, K+ (Brindley and Brown 1984)
Dioctahedral
Montmorillonite Oi^AL nH2 Mg O
y 2 y y 10
+
-_M ^OHh2 hSi4
Beidellite nH O Al
x 2 2 4 x x 10
+
-_M ^Si ^OHh2 iAl hO
Nontronite nH O Al
x 2 2
3+
4 x x 10 2
+
-_M ^Si ^OHh iFe hO
Trioctahedral
Saponite nH O Al
x y 2 3 y 4 x x 10 -
+
--_M ^Al,Fehyi^Si ^OHh2 i_Mg hO
Hectorite nH O Li O
y 2 3 y y 4 10 2
+
-_M ^OHh i^Mg hSi
to varying extents and can be exchanged with cations from
an external solution. The exchangeable cations contained
in smectites are most commonly Na+, Ca2+ and Mg2+
ions, but can include a wide range of other cations such as
Li+, K+, H+ and organic ions. The chemical composition
of smectites is therefore varied and complex due to vari-
ous possible cationic substitutions in both the tetrahedral
and octahedral sheets (Garcia-Romero et al. 2021 Al-Ani
and Sarapää 2008 Brindley and Brown 1984). Due to
the structure, presence of exchangeable cations, and small
crystal size (2 µm) of the smectite minerals, a number of
unique properties are observed. These properties include a
high cation exchange capacity, large swelling ability in the
presence of water, as well as the ability to strongly influence
the rheology of a liquid (Odom 1984).
The end-member compositions for the most com-
mon smectites are given in Table 1, where M+ represents
the exchangeable cations in the interlayer space. Although
these exchangeable cations are represented as monovalent
ions, equivalent numbers of ions of different valencies can
be substituted. The chemical formula for the smectites is
defined in terms of half a unit cell (i.e., ten oxygen atoms
and two hydroxyl ions). In half a unit cell, there are a total
of four tetrahedral sites and three octahedral sites. In diocta-
hedral smectites, such as montmorillonite, two of the three
octahedral sites are occupied in trioctahedral smectites, all
the octahedral sites are occupied. Intermediate chemical
compositions between the end-members in the dioctahe-
dral group, and between end-members in the trioctahedral
Silica tetrahedral sheet
Silica tetrahedral sheet
Alumina octahedral sheet
Interlayer space
O
H
Si ,Al
Al ,Mg
Na ,Ca ,Mg ,Li ,K ,H
Figure 1. Simplified structure of montmorillonite, adapted from Graham (2019)
Table 1. End-member compositions for the most common smectites, where M+ represents exchangeable
cations including Na+, Ca2+, Mg2+, Li+, K+ (Brindley and Brown 1984)
Dioctahedral
Montmorillonite Oi^AL nH2 Mg O
y 2 y y 10
+
-_M ^OHh2 hSi4
Beidellite nH O Al
x 2 2 4 x x 10
+
-_M ^Si ^OHh2 iAl hO
Nontronite nH O Al
x 2 2
3+
4 x x 10 2
+
-_M ^Si ^OHh iFe hO
Trioctahedral
Saponite nH O Al
x y 2 3 y 4 x x 10 -
+
--_M ^Al,Fehyi^Si ^OHh2 i_Mg hO
Hectorite nH O Li O
y 2 3 y y 4 10 2
+
-_M ^OHh i^Mg hSi