Due to its high
specific surface area and its unbalanced negative
electric charges,
clay is the most active mineral component of soil. It is a colloidal and most often a crystalline material. In soils, clay is a soil textural class and is defined in a physical sense as any mineral particle less than in effective diameter. Many soil minerals, such as
gypsum, carbonates, or quartz particles, are small enough to be classified as clay based on their physical size, but chemically they do not afford the same utility as do mineralogically defined
clay minerals. Chemically, clay minerals are a range of
phyllosilicate minerals with certain reactive properties. Before the advent of
X-ray diffraction clay was thought to be very small particles of
quartz,
feldspar,
mica,
hornblende or
augite, but it is now known to be (with the exception of mica-based clays) a precipitate with a mineralogical composition that is dependent on but different from its
parent materials and is classed as a
secondary mineral. The type of clay that is formed is a function of the parent material and the composition of the minerals in solution. Clay minerals continue to be formed as long as the soil exists. Mica-based clays result from a modification of the primary
mica mineral in such a way that it behaves and is classed as a clay. Most clays are
crystalline, but some clays or some parts of clay minerals are
amorphous. The clays of a soil are a mixture of the various types of clay, but one type predominates. Typically there are four main groups of clay minerals:
kaolinite,
montmorillonite-
smectite,
illite, and
chlorite. Most clays are crystalline and most are made up of three or four planes of
oxygen held together by planes of
aluminium and
silicon by way of
ionic bonds that together form a single layer of clay. The spatial arrangement of the oxygen atoms determines clay's structure. Half of the weight of clay is oxygen, but on a volume basis oxygen is ninety percent. The layers of clay are sometimes held together through
hydrogen bonds, sodium or potassium
ionic bonds and as a result will swell less in the presence of water. Clays such as
montmorillonite have layers that are loosely attached and will swell greatly when water intervenes between the layers. In a wider sense clays can be classified as: • Layer Crystalline
alumino-silica clays:
montmorillonite,
illite,
vermiculite,
chlorite,
kaolinite. • Crystalline Chain
carbonate and sulfate minerals:
calcite (CaCO3),
dolomite (CaMg(CO3)2) and
gypsum (CaSO4·2H2O). •
Amorphous clays: young mixtures of
silica (SiO2-OH) and
alumina (Al(OH)3) which have not had time to form regular crystals. •
Sesquioxide clays: old, highly leached clays which result in oxides of
iron,
aluminium and
titanium.
Alumino-silica clays Alumino-silica clays or
aluminosilicate clays are characterized by their regular
crystalline or quasi-crystalline structure.
Oxygen in ionic bonds with
silicon forms a
tetrahedral coordination (silicon at the center) which in turn forms sheets of
silica. Two sheets of silica are bonded together by a plane of
aluminium which forms an
octahedral coordination, called
alumina, with the oxygens of the silica sheet above and that below it.
Hydroxyl ions (OH−) sometimes substitute for oxygen. During the clay formation process, Al3+ may substitute for Si4+ in the silica layer, and as much as one fourth of the aluminium Al3+ may be substituted by Zn2+, Mg2+ or Fe2+ in the alumina layer. The substitution of lower-
valence cations for higher-valence cations (
isomorphous substitution) gives clay a local negative
charge on an oxygen atom Isomorphous substitution occurs during the clay's formation and does not change with time. •
Montmorillonite clay is made of four planes of oxygen with two silicon and one central aluminium plane intervening. The
aluminosilicate montmorillonite clay is thus said to have a 2:1 ratio of silicon to aluminium, in short it is called a 2:1 clay mineral. The seven planes together form a single crystal of montmorillonite. The crystals are weakly held together and water may intervene, causing the clay to swell up to ten times its dry volume. It occurs in soils which have had little
leaching, hence it is found in arid regions, although it may also occur in humid climates, depending on its mineralogical origin. As the crystals are not bonded face to face, the entire surface is exposed and available for surface reactions, hence it has a high
cation exchange capacity (CEC). •
Illite is a 2:1 clay similar in structure to montmorillonite but has potassium bridges between the faces of the clay crystals and the degree of swelling depends on the degree of weathering of potassium-
feldspar. The active surface area is reduced due to the potassium
ionic bonds. Illite originates from the modification of
mica, a
primary mineral. It is often found together with montmorillonite and its primary minerals. It has moderate
CEC. •
Vermiculite is a mica-based clay similar to illite, but the crystals of clay are held together more loosely by hydrated magnesium and it will swell, but not as much as does montmorillonite. It has very high CEC. •
Chlorite is similar to vermiculite, but the loose bonding by occasional hydrated magnesium, as in vermiculite, is replaced by a hydrated magnesium sheet, that firmly bonds the planes above and below it. It has two planes of silicon, one of aluminium and one of magnesium; hence it is a 2:2 clay. Chlorite does not swell and it has low CEC. •
Kaolinite is a very common, highly weathered clay, and more common than montmorillonite in acid soils. It has one silica and one alumina plane per crystal; hence it is a 1:1 type clay. One plane of silica of montmorillonite is dissolved and is replaced with
hydroxyls, which produces strong hydrogen bonds to the oxygen in the next crystal of clay. As a result, kaolinite does not swell in water and has a low
specific surface area, and as almost no
isomorphous substitution has occurred it has a low CEC. Where rainfall is high, acid soils selectively leach more silica than alumina from the original clays, leaving kaolinite. Even heavier weathering results in sesquioxide clays.
Crystalline chain clays The
carbonate and
sulfate clay minerals are much more soluble and hence are found primarily in desert soils where leaching is less active.
Amorphous clays Amorphous clays are young, and commonly found in recent volcanic ash deposits such as
tephra. They are mixtures of
alumina and
silica which have not formed the ordered crystal shape of
alumino-silica clays which time would provide. The majority of their negative charges originates from
hydroxyl ions, which can gain or lose a hydrogen ion (H+) in response to
soil pH, in such way as to buffer the soil pH. They may have either a negative charge provided by the attached hydroxyl ion (OH−), which can attract a cation, or lose the hydrogen of the hydroxyl to solution and display a positive charge which can attract anions. As a result, they may display either high CEC in an acid soil solution, or high anion exchange capacity in a basic soil solution.
Sesquioxide clays Sesquioxide clays are a product of heavy rainfall that has
leached most of the silica from alumino-silica clay, leaving the less soluble oxides iron
hematite (Fe2O3),
iron hydroxide (Fe(OH)3),
aluminium hydroxide gibbsite (Al(OH)3), hydrated manganese
birnessite (MnO2), as can be observed in most
lateritic weathering profiles of tropical soils. It takes hundreds of thousands of years of leaching to create sesquioxide clays.
Sesqui is Latin for "one and one-half": there are three parts oxygen to two parts iron or aluminium; hence the ratio is one and one-half (not true for all). They are hydrated and act as either
amorphous or
crystalline. They are not sticky and do not swell, and soils high in them (e.g.
lateritic soils) behave much like sand and can rapidly pass water. They are able to hold large quantities of phosphates, a
sorptive process which can at least partly be inhibited in the presence of decomposed (
humified) organic matter. Sesquioxides have low
CEC but these variable-charge minerals are able to hold anions as well as cations. Such soils range from yellow to red in colour. Such clays tend to hold phosphorus so tightly that it is unavailable for absorption by plants. ==Organic colloids==