2 : 1 unit presents only basal tetrahedral oxygens on its outer surfaces. Moreover,
the ionic bonding between K+in the interlayer site and the tetrahedral oxygens
is a relatively strong bond, making illitic clays stable minerals. This accounts for
their abundance as weathering products, particularly in temperate and colder
climates.
The smectite group of clay minerals are structurally similar to illites (Fig.
4.12). In octahedral sites, substitution of Al^3 +by Mg^2 +or Fe^3 +is common and
some substitution of Si^4 +by Al^3 +in the tetrahedral sites also occurs, resulting in
a net negative layer charge. This charge is, however, only about one-third the
strength of the illite layer charge. Consequently, smectite is not able to bond
interlayer cations effectively and the 2 : 1 units are not tightly bonded together.
This allows water and other polar solvents to penetrate interlayer sites, causing
the mineral to swell. Cations, principally H+, Na+, Ca^2 +and Mg^2 +, also enter the
interlayer site with water and neutralize the negative charge. Bonding between
the 2 : 1 units is effected by the hydrated cation interlayer, by a combination of
hydrogen bonds and van der Waals’ forces (Box 4.7). This weak bonding holds
cations loosely in the interlayer sites, making them prone to replacement by other
cations. As a consequence, smectites have high cation exchange capacity (Section
4.8).
The similar structure of illite and smectite allows mixing or interstratification
of 2 : 1 units to form mixed-layer clays. Most illites and smectites are interstrati-
fied to a small degree, but they are not classified as such until detectable by
X-ray diffraction. As one might expect, illite–smectite mixed-layer clays have
intermediate cation exchange capacity between the end-member compositions.92 Chapter Four
OHOHOHOH(0.96 – 1.8 nmor more)6 O
4 Si4 O + 2[OH]
4 AI
4 O + 2[OH]4 Si
6 OExchangeable cations and waterFig. 4.12The structure of a 2 : 1 clay mineral (smectite).