Oxides and Hydroxides
Many rocks, primary and secondary minerals contain ions such as silica, iron, aluminum, manganese, and / or smaller amounts of titanium. Oxides and hydroxides may be present as primary minerals (i.e., inherited from the parent material) or secondary / pedogenic minerals (i.e., formed as a result of soil genesis) in soils, whereas several processes are important to consider:
The release of ions such as Si4+, Al 3+, Fe2+, Mn 2+ out of minerals may occur due to protonation or oxidation. In the presence of protons (e.g. protons resulting from a reaction of CO 2 + H2O <- -> H + + HCO3 -) the silicate may break down by the following reaction (protonation) :
-SiO-Fe2+ + 2H+ <--> Fe2+ + 2HO-SI
The liberated Fe2+ may then be oxidized immediately after release if the environmental conditions are aerobic (oxygenated conditions), or it may migrate until it reaches an oxygenated zone or remain in a reduced state. The oxidation may occur (i) partly inside the minerals, which results in the release of the oxidized ion out of the mineral because the mineral becomes unstable, or (ii) in solution.
For example, the oxidation of Fe2+ (ferrous ion) to Fe3+ (ferric ion) can be written as:
Fe2+ + 3H2 O = Fe(OH)3 + 3H + + e-
Once released, iron hydrolyzes when it comes in contact with H 2O. The tendency for Fe2+ to hydrolyze and form hydroxides results from two characteristics of iron, i.e., its high affinity for the OH ligand, making the hydrated iron cation a strong acid, and its ready polymerization once hydrolysis proceeds. The resulting iron hydroxides have a low solubility and thus in a pH range > 3 are quite stable in terms of reverse hydrolysis. However, they are vulnerable to transformation in response to increasing reducing conditions:
FeOOH + e- + 3H + <--> Fe2+ + 2H 2O
This occurs (i) whenever oxygen becomes limited (because it has been used by aerobic microorganisms, or induced by water saturation, i.e. anaerobic environmental conditions), (ii) there is a source of organic matter and (iii) appropriate environmental conditions suitable for anaerobic microorganisms to facilitate electron transfer to Fe3+ as part of their metabolic process. Redox reactions are facilitated in soils by the activity of microorganisms (catalysis).
For a given redox couple, the position of the equilibrium depends on the locally prevailing value of pe, which is the negative logarithm of the electron activity, compared to pe0 of the redox couple. The latter value expresses the relative electron activity when reacting species are at unit activity. The aqueous electron is a useful conceptual device for describing the redox status of soils. Soil oxidizability can be expressed by the negative log of the free electron activity:
pe = -log(e-).
Large pe values favors electron-poor oxidizing species, whereas small pe values favors electron-rich reducing species.
Reduction processes are often expressed in terms of redox potential (Eh) measured in mV.
Eh = 0.059 pe (at 25° C)
A high redox potential equals to well-aerated environmental conditions and a low redox potential equals to saturated environmental conditions. Saturated soils become depleted of oxygen, because this is rapidly consumed by aerobic organisms and cannot be replenished by diffusion quickly. Then, anaerobic and facultative organisms continue the decomposition process. In the absence of oxygen, other electron acceptors begin to function, depending on their tendency to accept electrons. When flooding occurs the reduction of the remaining oxygen will take place first, followed by the reduction of nitrate, then manganese, iron, sulphate, and carbon dioxide (Figure 1.). The reduction of oxygen occurs by the O2 consumption of aerobic organisms, NO3 serves as a biochemical electron acceptor involving N-organisms that ultimately excrete reduced N, the reduction of Mn can be initiated in presence of NO 3-, whereas the reduction of Fe cannot be initiated in presence of NO 3 -, and sulfate reducing bacteria are involved to reduce SO 4 2-.
The oxides and hydroxides present in soils reflect the pedoenvironmental conditions of soil formation. The parent material, temperature, moisture, organic material, pH, and Eh control the formation of different types of oxides and hydroxides. Because the oxides and hydroxides, particularly of iron and manganese, show different colors they can be used as an indicator for processes of pedogenesis. It should be stressed that there are continual modifications of pedogenic processes acting in soils. Therefore, the soil properties which can be observed in the field, e.g. soil color expressed by the presence or absence of oxides and hydroxides and the distribution of them, also changes. Thus, it is also important to relate the oxides and hydroxides to contemporary processes of pedogenesis or soil formation associated to some previous periods.
Characteristics of oxides and hydroxides in soils are a relative high cation exchange capacity (CEC), which is due to the dissociation of protons from -OH and -OH2 groups of the hydroxides. This is true also for the oxides, which are often associated by -OH and -OH 2. The CEC for oxides and hydroxides is dependent on pH, where a high pH favors the H+ ions to dissociate from the functional groups and to replace the vacant places with cations. The oxides and hydroxides are efficient sorbents and sinks for:
In Figure 2 some soil morphological features associated with different redox statuses and drainage conditions are shown.
In horizons with concretions of hard nodules a 'c' is used for iron, aluminium, manganese, or titanium cemented nodules or concretions. A ' g' denotes gleying indicated by low chroma color (< 2), either total gleying or the presence of gleying in a mottled pattern. In illuvial horizons the accumulation of organic matter with or without sesquioxides (the oxides and hydroxides of iron and aluminium) are denoted by a 'h '. If the sesquioxide component contains enough iron so that color value and chroma exceeds 3, 'hs' is used. Generally, the accumulation of iron and the cementation (i.e., more than 90 % of the horizon is cemented) is denoted by 'sm'. Residual accumulation of sesquioxides after intense weathering is denoted by 'o'.
Primary minerals which contain iron are for example biotite, pyroxene, amphibole, and olivine. Iron oxides and hydroxides are formed by protonation and release of Fe ions out of primary or secondary minerals and / or oxidation. Their occurence provides useful information about soil formation.
Another important attribute of Fe is that its cationic charge is sensitive to changes in the redox status of the soil. This may impart clues via soil color about drainage conditions in the soil. Generally, in soils that do not have impeded drainage the majority of the Fe-oxides occur in the Fe 3+ state, which is typically associated with red, yellow or brown subsoil colors. With a progressive increase in impeded drainage conditions subsoil colors reflect an increasing influence of Fe 2+ on soil color. Fe2+ typically imparts a bluish gray color to poorly drained subsoils, often referred to as a gley color. Along this continuum of drainage conditions variations on patterns may be identified (so-called redoximorphic features (RMF)). In soils that are intermediate between well-drained and waterlogged, periodic reducing conditions may result in particular zones within the soil exhibiting variegated color patterns. These patterns consist of intricate combinations of reddish (high chroma RMFs: containing local concentrations of Fe 3+ oxides), whitish or light gray areas (low chroma RMFs: reflecting zones from which Fe has largely been removed) and sometimes bluish gray areas (gleyed zones: reflecting presence of Fe 2+ -oxides). The distinction between low chroma zones, which are local zones of Fe eluviation, and gleyed zones is not always simple in the field. Both conditions are indicative of reducing conditions, but represent differences in the extent of Fe 2+ translocation. Soil color and in particular patterns of redoximorphic features are used as field indicators of drainage conditions. The criteria are developed to suit local conditions and are ideally based on hydrological and chemical measurements, which are calibrated with observed morphology. This is particularly important because the distribution pattern of Fe-oxide colors may not necessarily reflect contemporary processes operating in the soil. In other words, some of the Fe-oxides still present in the soil may have formed under conditions quite different from those operating at present or they may reflect colors of inherited Fe-oxides or other minerals such as glauconite (greenish color that could be confused as a gley feature) that have not been significantly altered as a result of pedogenesis.
Where iron oxides are absence, soil color usually arise from uncoated mineral grains. They may occur evenly dispersed throughout the soil horizons or as concentrations in particular morphological features such as RMFs, nodules, pipestems. Sesquioxides are a term for the oxides of iron and aluminum and sesquioxic coatings (such as ferrans) can be formed by reduction and solution of Fe under anaerobic conditions and their subsequent oxidation and depositon in aerobic zones. If iron oxides (and manganese oxides) become concentrated in a soil horizon they may form cemented layers, called fragipans (denoted by a 'x'), which are hard to very hard and brittle when dry. In contrast, zones of Fe depletion are called neoalbans , which may occur in eluviated soil horizons. The term plinthite is used for B or C horizons (denoted by a 'v'), which are humus poor and iron rich. The material usually has reticulate mottling of reds, yellows, and gray colors and hardens irreversibly to ironstone hardpans or aggregates with repeated wetting and drying.
Figure 3. Soft and hard accumulations of iron and manganese in soil peds.
Iron oxides and hydroxides are very stable under aerobic conditions, but they become more soluble under anaerobic conditions (low redox potentials). They are able to form metal-organic complexes, where the metal cations are bonded by functional groups such as -COOH, =CO, -OH, -OCH 3, -NH2, -SH to organic compounds resulting in the formation of a ring structure incorporating the metal ion. These complexes are very stable and called chelates.
In Figure 4 a Eh-pH stability diagram for different iron oxides and hydroxides is shown. The diagram can be used to predict when a species may be oxidized or reduced. Reduction or oxidation can occur outside these boundaries, but only when mediated by an organism and at expense of metabolic energy.
Figure 4. Eh-pH stability diagram for iron oxides and hydroxides (Scheffer et al., 1989).
Several different Fe oxides and hydroxides can be distinguished, which differ in their crystal structure and various other properties (e.g. color, solubility, thermalbehavior). The basic unit of iron oxides and hydroxides ia the Fe(O,OH)6 octahedron. The variation between different iron oxides and hydroxides is mainly due to a variation in the arrangement of these octaheda. Following the 6 most widespread Fe oxides and hydroxides are described briefly:
Goethite (alpha-FeOOH): It is the most frequently occuring Fe-oxide in soil and has a characteristic yellowish brown color. In large concentrations it may also appear as dark brown or black. Goethite is found under a broad range of climatic and hydrological conditions and is the thermodynamically most stable of all Fe-oxides. A variety of conditions appear to favor its formation in preference to hematite, which include cool temperatures, moist soil conditions and presence of comparatively large amounts of organic matter. The absence of hematite and abundance of goethite in many soils of cool or temperate regions supports these general observations. Under suitable conditions, it has been suggested that goethite can form from any Fe source.
Hematite (alpha-Fe2O 3): It has a characteristic bright red color and primarily occurs in the better drained soils of warm temperate to tropical areas. It is also found in restricted areas of cool temperate areas either as a primary mineral or in more highly weathered soils, where it probably formed under climatic conditions warmer and drier than those operating at present. Its formation appears to be favored by warm dry conditions and small amounts of organic matter (OM). There are large areas of intertropics that have yellow topsoils (OM-rich) over red subsoils, which often contain yellow rims around root channels. This supports the concept of the OM-antihematic effect, i.e., hematite formation is hindered in the presence of comparatively large amounts of OM. Hematite probably forms from ferrihydrite (5Fe 2 O3*9H 2 O) through aggregation, dehydration, and internal structural rearrangement of the tiny ferrihydrite particles. This has been suggested because ferrihydrite has a hematite-like structure, except that it is highly disordered and is hydrated. As such, ferrihydrite is considered a necessary precursor for hematite formation.
Lepidocrocite (gamma-FeOOH): It has a characteristic bright orange color that is evident in soils where it occurs in large concentrations and is not masked by other pigments. It commonly occurs in association with goethite and usually in soils that have restricted drainage. It forms from the oxidation of Fe2+ compounds, which are common in wet soils. High partial pressure of CO 2 appears to favor goethite formation in preference to lepidocrocite. This has been demonstrated in laboratory experiments and is supported by field observations that show its virtual absence in calcareous soils, and preferential formation of goethite adjacent to roots and lepidocrocite concentrated further away in wet soils. Lepidocrocite formation also appears to be favored by slow oxidation rates and small concentrations of aluminium in the soil solution. Lepidocrocite is rarely found in very acid soils, which typically contain aluminium in the soil solution.
Maghemite (gamma-Fe2O 3): It is common in many soils, notably in the tropics and
subtropics and varies in color from red to brown. This oxide occurs in soils,
especially those derived from basic igneous rocks. Maghemites are commonly
concentrated towards the top of the soil profile. Several pathways have suggested
to account for their formation in soils, which include:
Magnetite: It is listed in most textbooks as a primary soil mineral that does not form through pedogenic processes. It occurs in soils as singular irregular black grains and is present in most soils. It is fairly resistant to weathering but may alter slowly to ferrihydrite, goethite or eventually hematite (via ferrihydrite). It has been documented the occurrence of magnetic bacteria in soils that contain minute crystals of magnetite. This the first account of in situ 'biogenic' formation of the mineral in soils.
Ferrihydrite (5Fe2O 3*9H2O): It is a common Fe-oxide found in soils and appears reddish brown in color if not masked by other pigments. It has a poorly ordered structure resembling that of hematite and was previously referred to as amorphous ferric hydroxide. Its formation is favored by rapid oxidation of Fe in the presence of large concentrations of OM and/or silicate. As such it occurs in limited amounts in tropical soils that do not have these conditions. Phosphate anions also appear to favor its formation in preference to other Fe-oxides. The presence of ferrihydrite generally indicates that conditions are not favorable to crystal growth. However, it is generally considered to be a 'young' Fe-oxide that will on a pedogenic time scale eventually transform to a more stable and more crystalline Fe-oxide (i.e., crystal-inhibiting substances become degraded, translocated etc., allowing for its transformation).
Some other important iron compounds found in soils are:
Pyrit (FeS): It is found in waterlogged soils, i.e. under reducing environmental conditions. The color of FeS is black.
Table 1. Summary of iron oxides and hydroxides and their pedoenvironments.
The aluminum oxides and hydroxides have a non-distinctive grayish-white color, which is easily masked in soils except when large concentrations occur. In acid soils, deposits of amorphous aluminium hydroxide form in the interlayers of expanding lattice clays, and occur as surface coatings on clay minerals generally. This amorphous material slowly crystallizes to gibbsite (gamma-Al(OH)3), the principal aluminium hydroxide in soil, which is very stable material. This may become dehydrated to form boehmite (alpha-AlOOH), which is common in bauxite deposits but less common in soils. Gibbsite accumulates in old soils that are in an advanced stage of weathering and in younger soils of the tropics.
Al3+ in solution hydrolyzes to produce H+ as follows:
Al3+ + H2 O <--> Al(OH)2+ + H +
The hydroxy aluminium ion may also hydrolyze:
Al(OH)2+ + H2 O <--> Al(OH)2 +1 + H+
Thus, the major source of H+ in moderately and strong acidic soils is aluminium hydrolysis. The Al 3+ cation has a higher charge than other cations such as K +, Na+, Ca 2+, or Mg2+, and in soils with pH <5 the cations associated with minerals are replaced by Al 3+ rather than by H3O +. The ability to replace cations is smaller for H 3O+ in comparison to Al 3+.
The weathering of primary minerals, such as biotite, pyroxene, amphibole, containing Mn2+, produce in an aerobic environment brown/black Mn4+. The reaction can be written as:
Mn2+ + H2 O = MnO2 + 4H+ + 2e-
(MnO2) is a very
stable manganese oxide. Often manganese is associated with other ions such
as Ba, Ca, K, Na, Li, NH4, Co, Cu and
Ni, therefore the manganese oxides and hydroxides have variable forms. For
example, birnessite (Na,Ca,K,Mg,Mn2+ )Mn 64+ O 14 *H2 O, lithiophorite (LiAl 2Mn 2+ Mn 2 4+ O9 *3H 2 O), or hollandite (BaMn 8 O 16). Manganese oxides show even a greater
tendency than iron oxides to occur in concretions. A reason might be the
reduction of Mn 4+ to Mn 2+, which is relatively soluble, more readily soluble than
for example Fe2+ . Manganese-rich micromorphological
zones in peds that are black, often also contain large amounts of iron oxides.
However, iron-rich concretions have been shown to be low in manganese oxides.
Manganese oxide minerals have a black color, which is sometimes difficult
to distinct from the black color or organic material.
Sulphates: Gypsum (CaSO4 *2H 2O) which is common in soils (-> fertilizers) and jarosite (KFe3 (OH)6 (SO4 )2 which is rarely found in soils are sulphates. An accumulation of gypsum in a B or C horizon is denoted with a 'y'. If there is a cementation of more than 90 % and roots can penetrated only through cracks, the symbol 'ym' is used.
Chlorides: The chloride halite (NaCl) is found in soils formed in marine or lacustrine deposits. The accumulation of sodium is denoted by a 'n', indicating a high level of exchangeable sodium in the horizon.
Carbonates: Primary minerals which contain Ca 2+ are calcite (CaCO3 ), dolomite (Ca,Mg)CO3, plagioclase, pyroxene, and amphibole. They are easily weatherable and Ca2+ can be released. The solubility of Ca2+ is dependent on the CO 2 partial pressure (pCO2 ). The higher the pCO2 the higher the pH (pH = -0.67 lg pCO2 + 7.23, where pCO 2 is in kPa) A pH < 7 will be reached if all carbonate is dissolved and begins to leach or migrate. Carbonates have a characteristic to stick mineral particles and organic compounds together to form aggregates. Therefore, carbonates improve soil structure, particularly in A and B horizons. An accumulation of carbonates, usually calcium carbonate, in a horizon is designated with a 'k'. If more than 90 % of the horizon is cemented by calcium and roots can penetrate only through cracks, the symbol 'km ' is used. If carbonates are leached from the A or even B horizon those layers become acid to very acid. This alters the soil structure, cation exchange capacity, biological activity, etc.
Phosphates: A primary mineral which contains P is apatite (Ca 4(CaF or CaCl)(PO 4) 3. Apatite is stable in non acid soils but when the pH drops below 7 apatite is weathered fast. Orthophosphates (H 2PO 4- and HPO 42- ) that are released by mineralization is rapidly adsorbed by the soil particles. This process is called phosphate fixation because the process is difficult to reverse. Phosphate on surfaces that can be readily desorbed and phorphate in solution is called labile phosphate. In contrast, the P held in insoluble compounds or organic matter is called non-labile phosphate (P occluded in surface oxides and insoluble P compounds). Organic P, which is the major source of P for the soil microorganisms and mesofaunas is rapidly mineralized and / or immobilized by the microorganisms, especially bacteria, which have a relative high P requirement. Bacterial phorphate residues comprise mainly the insoluble Ca, Fe, and Al salts of inositol hexaphosphate, the phytates.