Oxidation soil weathering

The B soil horizon largely consists of clay minerals, iron (oxy)(hydr)oxides, and/or calcite. The C horizon, which is composed of sediments and weathered bedrock, usually occurs below the B. Arsenic in oxidizing soils readily sorbs and/or coprecipitates with iron and other (oxy)(hydr)oxides in B and C horizons (Reynolds, Naylor and Fendorf, 1999 Lund and Fobian, 1991). Below the C is the R horizon, which is unweathered bedrock. 

Ritsema, C. J. and J, E. Groennenberg. 1993. Pyrite oxidation, carbonate weathering, and gypsum formation in a drained potential acid sulfate soil. Soil Sci. Am. J. 57 968-976. 

Soils that have been contaminated by mine wastes, tailings, smelter slags, or smelter particulates can contain a complex mixture of minerals present in the soils prior to contamination minerals contributed by the contaminants minerals formed by soil weathering, biological reactions, and chemical reactions with infiltrating waters and soil moisture windblown dust, and other anthropogenic materials (Ruby et al, 1999). For example, reactions of lead oxide with soil moisture in alkaline soils can precipitate lead carbonate, whereas reactions in acidic soils can precipitate lead sulfate. 

Uranium U(VI) minerals are most often products of the oxidation and weathering of nearby primary U(IV) ore minerals such as uraninite [U02(c)I and coffinite [USi04(c)l (cf. Pearcy et al. 1994). They also form by evaporative concentration of dissolved U(VI), particulary under arid conditions. Schoepite (/J-UOj 2H2O) is fairly soluble and, therefore, is a rare mineral, whereas carnotite K2(U02)2(V04)2j and tyuyamunite (Ca(U02)2(V04)2j, which have lower solubilities (particularly above pH 5) are the chief oxidized ore minerals of uranium. The plots in Figs. 13.5 and 13.6 indicate that uranyl minerals are least soluble in I0W-CO2 waters, and, therefore, are most likely to precipitate from such waters. This is con.sistent with the occurrence of carnotite and tyuyamunite in oxidized arid environments with poor. soil development (Chap. 7), such as in the calcrete deposits in Western Australia (cf. Mann 1974 Dall Aglio et al. 1974), and in the sandstone-hosted uranium deposits of the arid southwestern United States (cf. Hostetler and Carrels 1962 Nash et al. 1981). The... 

Ultisols form on relatively old geologic terranes, where abundant precipitation produces deeply weathered soils. Extensive leaching and warm soil temperatures over prolonged periods result in rapid and nearly complete alteration of weatherable minerals into secondary clays and oxides. Soils in the study site are classified as Paleudults, in which "udult" refers to the suborder of Ultisols and "pale" means "old development". Udult soils have low organic-matter content. They form in humid climates where dry periods are of short duration and the water table remains below the solum throughout most of the year (21). 

As soils weather and Si, Ca, Mg, Na, and K are leached away, the soil s colloidal fraction becomes enriched with Al, Fe, Mn, and Ti oxides and hydroxyoxides. The structural organization of these hydroxyoxides ranges from amorphous to crystalline. These Al, Fe, and Ti oxides and allophane are prominent nonlayer silicate minerals in most soils and then- content in soils increases with increased weathering. 

The Ti oxides commonly found in soils and clay sediments are rutile and anatase, both TiC>2 and inherited from the parent rock. Because Ti oxides resist weathering so strongly, they are often used as indicators of the original amount of parent material from which a soil has formed. 

Heavy metals and metalloids are selectively adsorbed on variable-charge minerals (e.g., Al, Fe and Mn oxides), which occur in soils in advanced stages of weathering and in rhizosphere soils. Weathering induced hy LMMOAs and other biomolecules facilitates the release of Fe and Al from primary and secondary minerals, promoting the formation of Fe and Al oxides, usually of short-range order. 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

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. 

As a soil develops, OM decomposes to produce humus, which is black. Additionally, release of iron from minerals by weathering yields various reds and yellows. Both mechanisms yield soil coloring agents. Under oxidizing conditions, where soil is not saturated with water, the iron will be oxidized and thus in the ferric state [Fe(III)]. When the iron and OM are deposited on the surfaces of sand, silt, clay, and peds, they develop a coat that gives them a surface color. However, soil color is not only a surface characteristic but extends through the soil matrix. Under oxidizing conditions, soil has a reddish color. The chroma of this color depends to some extent on the amount of and the particular iron oxide present. 

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