Secondary minerals soils

Table 4 Structures of common secondary soil minerals...

All of the organic matter and part of the aluminosilicates, hydroxyoxides, and silica in soil exist in structures too small or too poorly crystalline to be detectable by x-ray diffraction, These amorphous materials are not well understood, but they should logically be among the most reactive of soil components, because their structure is so open and their surface area so great, They represent a transition state between unweathered parent materials and well-crystallized secondary soil minerals. 

The inorganic component of soil is dominated by four elements O, Si, Al, and Fe (Jackson, 1964). Together with Mg, Ca, Na, and K they constitute 99% of the soil mineral matter (see Table 8-2). Minerals in soil are divided into primary and secondary minerals. Primary minerals, which occur in igneous, metamorphic, and sedimentary rocks, are inherited by soil... 

Nordstrom, D. K., 1982, Aqueous pyritc oxidation and the consequent formation of secondary iron minerals. In Acid Sulfate Weathering. Soil Science Society of America Special Publication 10, 37-56. 

Transport of contaminants by surface runoff is illustrated in the experimental results of Turner et al. (2004), which deal with the colloid-mediated transfer of phosphorus (P) from a calcareous agricultural land to watercourses. Colloidal molybdate-reactive phosphorus (MRP) was identified by ultrafiltration associated with particles between l am and Inm in diameter. Colloidal P compounds can constitute a substantial component of the filterable MRP in soil solution and include primary and secondary P minerals, P occluded or adsorbed on or within mineral or organic particles, and biocolloids (Kretzschmar et al. 1999). 

With all batch techniques, there is the common problem of not removing the desorbed species. This can cause an inhibition of further adsorbate release (Sparks, 1985, 1987a), promote hysteretic reactions, and create secondary precipitation during dissolution of soil minerals (Chou and Wollast, 1984). However, one can use either exchange resins or sodium tetraphenylboron, which is quite specific for precipitating released potassium, as sinks for desorbed species and still employ a batch technique (Sparks, 1986). Also, since in most batch methods the reverse reactions are not controlled, problems are created in calculating rate coefficients. This is particularly true for heterogeneous systems such as soils. 

The inorganic minerals of soil are classified into (a) primary minerals and (b) secondary minerals (Table 3.3). Primary minerals are minerals with the chemical composition and structure obtained during the crystallization process of molten lava, whereas secondary minerals are those that have been altered from the original structure and chemical composition by weathering, a process referred to as the geomorphic cycle (Fig. 3.2). Generally, the size of soil mineral particles varies from clay-sized colloids (< 2 pm) to gravel (< 2 mm) and rocks. 

In the secondary lands of Paragominas, we have measured the soil solution concentrations from tension lysimeters at 0.25, 3.0, and 7.0 m. Tension lysimeters tend to collect soil waters that have Infiltrated slowly through the soil profile and are in more intimate contact with soil minerals. For our elements of interest, P and K, there is a general decrease in the concentration of these elements with depth (Table 9.3). At 7.0 m depth, soil solution concentrations are less than or equivalent to those input in bulk... 

The differentiation of soil permeability when soil becomes clogged with clay and mineral precipitation causes lateral flow of saline soil water and shallow groundwater towards low-lying areas. The final stage of the dryland cycle is salinization of adjacent streams and rivers. The chemical composition of the salinized river in the dryland environment reflects the net results of salt recycling between soil, subsoil, groundwater, secondary soil, soil solution, and surface water (Figure 10). 

Waste piles from former bedrock mining in the area are found to contain up to 30% As, the majority in secondary arsenate minerals, particularly scorodite (Williams et ah, 1996). Alluvial soils also contain up to 5000 mg kg As. In these, Fordyce et al. (1995) concluded that some 20% of the As was present in crystalline iron oxides, with the remainder assumed to be in sulphate mineral phases or other oxidised products. 

Figure 5.12 depicts a hypothetical soil pH-buffer capacity curve. It is clear from this relationship that the soils most likely to reach very low pH and toxic Al solubilities even with modest inputs of acidity are those with low inherent buffer capacities (arising from the lack of carbonates, clay, or humus content) or those whose buffers have already been expended in neutralizing past acid inputs. These are termed acid-sensitive soils, and are typically acid soils with little clay and humus. They are the most likely soils to reach extremely low pH (<4) on exposure to acid rain or other sources of acidity. Since primary and secondary aluminosilicate mineral dissolution is very pH sensitive, phytotoxic concentrations of soluble AP" are increasingly likely as the pH lowers. 

The concentration of magnesium in soils generally lies in the range between 0.5 g/kg for sandy soils and 5 g/kg for clay soils. The levels of magnesium are higher in clay soils due to the presence of weatherable ferromagnesian minerals, such as biotite, serpentine, and olivine and also the carbonate mineral dolomite. It is also present in secondary clay minerals, such as chlorite and vermiculite. 

Magnesium is an important constituent of many primary and secondary aluminosilicate minerals (with the exception of the feldspars). Magnesium in mafic (Mg2+- and Fe2+-rich) minerals often leads to the formation of chlorite and mont-morillonite clay minerals in soils. 

Minerals of the clay fraction of soils are largely secondary, that is, formed by low-temperature reactions and either inherited from sedimentary rocks or formed directly in the soil by weathering. These secondary (authigenic) minerals in soils commonly... 

---