Wider controls on soil and clay mineral formation

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 & 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This gibbsitic unit may then have SiO4 tetrahedra adsorbed on to it to form clay mineral structures.

A minority of unweathered rock-forming silicates, for example the micas, are already sheet silicates. It is not difficult to envisage that alteration could trans form these to clay minerals. Alteration is most likely in the interlayer areas, especially at damaged crystal edges. The clay mineral formed will depend on the composition of both the original mineral and the ions substituted during alteration. For example, the replacement of K+ in muscovite by Mg2+ would lead to the formation of magnesium smectite.

Topography influences most of the soil-forming factors (Section 4.6). Where all the soils in an area of varied topography have formed on the same parent material, the soils differ principally due to changes in relief and drainage. These soils are described as part of a catena ('chain') and are intrinsically linked to the landscape of a region or area. Two key processes control the development of a soil catena: (i) erosion and subsequent transport and deposition of eroded material; and (ii) leaching and subsequent transport and deposition of dissolved materials. The latter process is dependent on soil chemistry and in the tropics where rainfall is high, a chemical catena develops.

Wet tropical environments promote dissolution and transport of a number of soil constituents. Chemical species have been categorized into four groups by their relative mobility during rock weathering (Table 4.8). At high altitude in the tropics, high rainfall causes mobilization of Group I soluble ions and oxyanions from the soil. These easily dissolved species (see Section 5.2) are carried in solution down slope to lower altitude where they accumulate. Thus, the tropical catena is characterized by distinct soil end-members (Fig. 4.16). The high-altitude end-member is iron- and silica-rich Groups III and IV and base cation-deficient (Box 4.11), and called an oxisol or ferralsol depending on the classification system used (Plate 4.1, facing p. 138). By contrast, the low altitude end-member is base cation-rich and also clay-rich and known as a vertisol (swelling clay soil).

The high-altitude oxisols (ferralsols) are of low fertility on account of their low cation exchange capacity (CEC) (Section 4.8). Furthermore, the high iron concentrations can prove toxic to some plants and animals while the formation

Table 4.8 Mobility of different chemical species in relation to rock weathering. Modified from Polynov (1937).

Species

Mobility*

Comments'

Group I

Cl-

100

Soluble anions easily leached by water

SO42-

57

Group II

Ca2+

3.00

Relatively soluble cations, easily leached by water

Na+

2.40

Mg2+

1.30

K+

1.25

Group III

SiO2

0.20

Relatively insoluble element, typically present as

quartz grains

Group IV

Fe2O3

0.04

Highly insoluble elements present as Fe and Al

Al2O3

0.02

oxides

^The reasons for differing solubility are discussed in Section 5.2 and depicted in Fig. 5.2.

^The reasons for differing solubility are discussed in Section 5.2 and depicted in Fig. 5.2.

Red oxisols (feralsols) (kaolinitic, Fe- and Al-rich)

Laterite

Red oxisols (feralsols) (kaolinitic, Fe- and Al-rich)

Laterite

Base cations Smectite clays

Dark vertisols

Base cations Smectite clays

Dark vertisols rrf^

(Smectitic, base cation-rich)

1 Low cation exchange capacity 1 Low fertility

1 Residual Fe and Al can be toxic to plants

1 High cation exchange capacity 1 High fertility

1 Deep soil cracking can affect agriculture in dry seasons

Fig. 4.16 Diagram illustrating the soil end-members of a tropical catena.

Box 4.11 Base cations

The term base cation, or non-acid cation, is often used in soil chemistry and refers to cations of the alkali metals and alkali earth metals (see Section 2.2), most importantly Ca2+, Na+, K+ and Mg2+. The weathering of these metals from crustal minerals is a slow, but very important process that helps neutralize acidity. For example, weathering of the (Na)-rich feldspar, albite, proceeds as in equation 1.

The acidity (H+) contained in H2CO3 is neutralized and dissolved cations (Na+), bicarbonate (HCO-) and silicic acid (H4SiO4) are released. These 'released' base cations thus become available in soilwater to take part in exchange reactions (Section 4.8).

2NaAlSi3O8(s) + 9H2O(l) + 2H2COB(aq) ^ Al2Si2O5(OH) )+ 2Nafaq) + 2HCO-(aq) + 4H4SiO4(aq)

of impenetrable siliceous-iron layers (laterite) inhibit plant growth. By contrast, the low-altitude vertisols have high fertility on account of their high CEC. These clay-rich soils do not suffer from metal-associated toxicity, although deep cracking during dry periods can be a problem for agriculture.

The marked contrast in solubility (Box 4.12) between insoluble oxides or oxy-hydroxides of aluminium and iron and other more soluble soil cations and H4SiO4

Box 4.12 Solubility product, mineral solubility and saturation index

The dynamic equilibrium between a mineral and its saturated solution (i.e. the point at which no more mineral will dissolve), for example:

CaCO

3(calcite)

eqn. i is quantified by the equilibrium constant (K), in this case:

aCa2+. aCO3 aCaCO3

pure water. The case for calcite is simple since each mole of CaCO3 that dissolves produces one mole of Ca2+ and one mole of COf-. Thus:

Calcite solubility = aCa2+ = aCO^ and therefore:

Since the CaCO3 is a solid crystal of calcite, it is difficult to express its presence in terms of activity (see Section 2.6). This is overcome by recognizing that reaction between a solid and its saturated solution is not affected by the amount of solid surface presented to the solution (as long as the mixture is well stirred). Thus the activity of the solid is effectively constant; it is assigned a value of 1 or unity (see eqn. 3), and makes no contribution to the value of K in equation 3.

The equilibrium constant for a reaction between a solid and its saturated solution is known as the solubility product and is usually given the notation Ksp. Solubility products have been calculated for many minerals, usually using pure water under standard conditions (1 atm pressure, 25°C temperature).

The solubility product for calcite (eqn. 1) is thus:

Ksp = aCa2+.aCO3- = aCa2+. aCO3- = 3.3 x 10-9 mol2l-2 1

The solubility product can be used to calculate the solubility (mol l-1) of a mineral in

Thus:

Calcite solubility = ^3.3 x io-9 = 5.7 x io-5 mol I-1

The degree to which a mineral has dissolved in water can be calculated using the saturation index, i.e.:

Degree of saturation = O = IAP

Ksp eqn. 7

IAP is the ion activity product, i.e. the numerical product of ion activity in the water. An W value of 1 indicates saturation, values greater than 1 indicate supersaturation and values less than 1 indicate undersaturation.

For example, groundwater in the Cretaceous chalk aquifer of Norfolk, UK, has a calcium ion (Ca2+) activity of 1 x 10-3mol l-1 and a carbonate ion (CO!-) activity of 3.5 x 10-6mol l-1. The saturation state of the water with respect to calcite is:

i.e. the water is slightly supersaturated with respect to calcite.

Table 4.9 Chemical index of alteration (CIA) values for various crustal materials. Data from Nesbitt and Young (1982), Maynard et al. (1991) and Taylor and McLennan (1985).

Material CIA

Clay minerals

Kaolinite 100

Chlorite 100

Illite 75-85

Smectite 75-85

Other silicate minerals

Plagioclase feldspar 50

Potassium feldspar 50

Muscovite mica 75

Sediments

River Garonne (southern France) suspended load 75*

Barents Sea (silt) 65*

Mississippi delta average sediment 64*

Amazon delta muds 70-75

Amazon weathered residual soil clay 85-100

Rocks

Average continental crust (granodiorite) 50

Average shales 70-75

Basalt 30-40

Granite 45-50

*Value calculated using total CaO rather than CaO* (see text).

under normal soil pH ranges (Table 4.8) has been used to formulate a chemical index of alteration (CIA); using molecular proportions:

CIA = ((Oj/A^Oj + CaO* +Na2O3 + K2O) 1000 eqn. 4.17

where CaO* is CaO in silicate minerals (i.e. excludes Ca-bearing carbonates and phosphates). Thus, CIA values approaching 100 are typical of materials formed in heavily leached conditions where soluble calcium, sodium and potassium have been transported away from the weathering site. Kaolinite clays attain such values (Table 4.9), whereas illites and smectites have CIA values around 75-85 (Table 4.9). In comparison, unleached feldspars have CIA values around 50.

The CIA predicts that kaolinite will form under heavily leached conditions, and this is confirmed by observations in tropical weathering regimes. On stable well-drained land surfaces where weathering and leaching have been prolonged, the oxisols (ferralsols) develop kaolinitic and, in extreme cases, gibbsitic clay mineralogies (Fig. 4.17). Such sites are mantled by iron-rich (laterite) and aluminous (bauxite) surface deposits (Plate 4.1). These surface deposits can become thick enough to prevent further interaction between surface waters and bedrock, lowering the rate of subsequent bedrock weathering.

tion2700 at

5 2100

Tundra

Taiga-spodosol

Semidesert and desert

Steppes

Savannah

Tropical forest Savannah

Tundra

Taiga-spodosol

Semidesert and desert

Steppes

Savannah

Tropical forest Savannah zone

Moderate amounts of soil organic matter promote weathering reactions

Large amounts of soil organic matter promote aggressive weathering conditions

Moderate amounts of soil organic matter promote weathering reactions z

Rock debris - little alteration chemically pT^ Illite - smectite clay minerals yzza formed by acid hydrolysis

_ Kaolinite clay minerals

_| formed by hydrolysis plus leaching

Large amounts of soil organic matter promote aggressive weathering conditions i Gibbsite formed by ' excess leaching

Laterites and bauxites formed as residua in heavily leached conditions

Z Fresh rock zone

Fig. 4.17 Schematic relationship between weathering zones and latitude-vegetation-climatic zones. The influence of relief (mountainous areas), where soils are typically thin, is not included. After Strakhov (1967). With kind permission of Kluwer Academic Publishers.

By contrast, smectite clays develop in poorly drained sites. On the basaltic island of Hawaii, soil clay mineral type changes in the sequence smectite-kaoli-nite-gibbsite as rainfall amount increases (Fig. 4.18). A similar, generalized zona-tion has been proposed for clay mineral distribution with depth in soils, again based on the degree of leaching (Fig. 4.19).

Intense leaching favours kaolinite formation since cations and H4SiO4 are removed, lowering the silicon: aluminium ratio and favouring the 1: 1 structural arrangement. Less intense leaching favours a higher silicon: aluminium ratio, allowing the formation of various 2 : 1 clay minerals, depending on the supply of cations. For example, the weathering of basalt provides abundant magnesium for the formation of magnesium smectite. In the most intense tropical weathering environments, all of the silica is removed, favouring the formation of gibbsite, which can be thought of as a 0:1 arrangement (i.e. only octahedral sheet present; Fig. 4.19).

Clay Minerals Formation

Fig. 4.18 The influence of climate on clay mineralogy in Hawaii. The relatively rapid water flow rates associated with high rainfall result in the preferential removal of cations and silica.

After Sherman (1952).

Fig. 4.18 The influence of climate on clay mineralogy in Hawaii. The relatively rapid water flow rates associated with high rainfall result in the preferential removal of cations and silica.

After Sherman (1952).

Fe, Al oxides

Depth

Fe, Al oxides

Gibbsite Kaolinite

Fresh basalt+ substrate

Clay mineral

Gibbsite Kaolinite

Leaching

Smectite

Fig. 4.19 Idealized vertical distribution of clay minerals formed under leaching conditions in soils developed on basalt. CIA values increase from 30-40 in fresh rock to near 100 in heavily leached surface soils.

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