Agriculture Reference
In-Depth Information
Other minerals in this group are
imogolite
and
allophane
, most commonly
found in relatively young soils (1000-2000 years) formed on volcanic ash and
pumice. The Si:Al mole ratio ranges from 0.5 in imogolite to 1 in some of the
allophanes. These minerals form hollow tubes (imogolite) or spheres (allophane)
between 1 and 5 nm in diameter. Because they are hollow, these minerals have
very high specific surface areas, encompassing both internal and external crystal
surfaces (table 2.1).
Isomorphous substitution of Al
3
for Si
4
in these minerals is variable, rang-
ing from a few percent in kaolinite to 50% in some allophanes. This substitution
gives rise to a negative lattice charge, measured in moles of charge (box 2.5). The
lattice charge is neutralized when cations in solution are attracted to the clay sur-
face. A charge within the lattice is considered
permanent
because it depends only
on the crystal structure. In addition, a
variable
charge develops at the crystal edge
faces due to the association or dissociation of protons at exposed O and OH
groups. The combination of charge effects—one due to substitutions within the
crystal lattice and the other due to unsatisfied valencies at crystal edge faces—de-
termines the
cation exchange capacity
(
CEC
) of a clay mineral (section 4.6.1).
Minerals with a Si:Al Mole Ratio of 2
Clay minerals in this group all have 2:1 layer-lattice structures. They differ mainly
in the extent and location of isomorphous substitution in the crystal lattice. The
main isomorphous substitutions are Al
3
for Si
4
in the tetrahedral sheets and
Mg
2
and/or Fe
2
for Al
3
in the octahedral sheets. The predominant location
of the substitution determines the type of cation that is attracted to the mineral
to become an interlayer cation in the crystal. The physicochemical reasons for this
are discussed in box 2.6. The end result of these ionic substitutions is three main
2.2.4.2
Box 2.5
Moles of Charge and Equivalents
The molar mass of an element is defined as the number of grams weight per
mole (abbreviated to mol) of the element. The standard is the stable C-12 isotope
of carbon. On this scale, H has a molar mass of 1 g, K a mass of 39 g, and Ca a
mass of 40 g. The recommended unit of charged mass for cations, anions, and
charged surfaces is the
mole of charge
, which is equal to the molar mass divided by
the ionic charge. Thus, for the elements H, K, and Ca, the mass of a mole of
charge is
H
1 g
1/1
K
39 g
39/1
Ca
2
20 g
40/2
For clay minerals and soils, the most appropriate unit of measurement is the
centimol (cmol) of charge
(
)
or
(-)
per kg
. In the older soil science literature, the
charge on ions and soil minerals was expressed in terms of an
equivalent weight
,
which is the atomic mass (g) divided by the valency (and identical to a mole of
charge). The
CEC
of a mineral was expressed in milliequivalents (meq) per 100 g,
which is numerically equal to cmols charge/kg.
