Organic matter silicate weathering

Generally for sedimentary rocks substitution in silicate lattices of primary minerals is of lesser importance. As the primary minerals decompose during weathering the alkaline earths and alkali metals tend to remain in solution and some of the metallic micronutrients pass into the lattices of the secondary or clay minerals. Others become adsorbed onto clay sized particles, are incorporated into humified organic matter by complexation or separate as precipitates following changes in redox potential. 

Soil is the product formed when the rocks of the earth s crust are exposed at the surface and are subjected to various physical, chemical, and, eventually, biological weathering processes. The minerals in these rocks are predominantly silicates, which dominate the characteristics of most soils. Table 1 shows those elements that are found in the crust above an average concentration of 1% and their corresponding soil content. The importance of aluminosilicates in soil is clear from the enrichment factors of approximately 1 for O, Si, and Al. Some loss occurs of K, Fe, Ca, Na, and Mg as a result of soil processes. But two elements, C and N, show considerable enrichment in soil because of the crucial role played by organic matter. 

Of course, once the ore is obtained from its deposit, the actual work of extracting the desired metal has yet to be accomplished. In addition to metals, a variety of other substances comprise natural minerals. Since aluminum and silicon are the most prevalent elements in the Earth s crust, most of the metals exist naturally as aluminates, silicates, or aluminosilicates. The most common minerals are feldspars and clays. These materials have been used since ancient times for the production of materials such as pottery, brick, and china. An example of a feldspar is K2Al2Si60i6, which corresponds to a mixture of potassium superoxide, alumina, and silica (K20-Al203 6Si02). Upon contact with water and carbon dioxide, a weathering reaction results in kaolinite, an aluminosilicate clay (Eq. 1). However, in addition to these oxidized sources of metals, there are substances such as alkaline carbonates, sulfates, phosphates, as well as organic matter that need to be removed to yield the desired metal. As you would expect, the yield for this process is quite low ores typically possess less than 1 % of the desired metal ... 

On a time series of Quaternary marine terraces in northern California, Brimhall et al. (1992) conducted the first mass balance analysis of soil formation over geologic time spans. This analysis provided quantitative data on well-known qualitative observations of soil formation (i) the earliest stages of soil formation (on timescales of 10 -10 yr) are visually characterized by loss of sedimentary/rock structure, the accumulation of roots and organic matter, and the reduction of bulk density and (ii) the later stages of soil development (>10 yr) are characterized by the accumulation of weathering products (iron oxides, silicate clays, and carbonates) and the loss of many products of weathering. 

These together with the burial of organic matter are the fundamental weathering reactions that control atmospheric CO2 over geological time (Ebelmen, 1845 Urey, 1952). Carbon dioxide is transferred from the atmosphere to carbonate rocks. The weathering of potassium and sodium silicates does not have a direct effect on CO2, because sodium and potassium are not removed from seawater as carbonates, but rather as silicates (Mackenzie and Garrels, 1966). These reactions return the CO2 to the atmosphere. 

Fine clastic sediments, mostly mudrocks, in contrast to their coarser counterparts, are either derived by first cycle weathering of silicate minerals or glass, or from recycling of older mudrocks. Physical comminution plays only a secondary role. The average shale is composed of 40-60% clay minerals, 20-30% quartz, 5-10% feldspar and minor iron oxide, carbonate, organic matter, and other components (Yaalon, 1962 Shaw and Weaver, 1965). Granitic source rocks produce shales richer in kaolinite and illite, the... 

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 Si04 tetrahedra adsorbed on to it to form clay mineral structures. 

Most soil particles have a net negative charge because it is the dominant charge on layer silicates and organic matter. However, some highly weathered soils and soils dominated by volcanic ash containing aiiophane and hydrous oxides may actually have a net positive charge. 

Silicate weathering resulting from the dissolution of ambient atmospheric C02 would be slow under the low carbonic acid concentrations produced at present-day atmospheric C02 levels. However, much higher C02 concentrations are found in soil pore-waters, due to the respiration of plant roots and the microbial oxidation of organic matter. Organic acids from root exudates and microbial activity also contribute to the weathering process (Berner 1992). 

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