Quartz weathering soils

Silicon reactions are central to rock weathering and soil development. Silicon is the soil component lost in greatest amount from rock minerals during weathering, and the transformations of silica into secondary minerals are the major reactions of soil development. The sand fraction of soils is usually >90% quartz (SiC>2), the most prevalent form of Si in soils. Highly weathered soils may contain as little as 20% Si (Table 2.1a). A1 and Fe ore deposits are essentially highly weathered soils from which most of the Si has been lost. 

Extensive, horizontal sandstone plateaus occur in tropical shield areas. Well-known examples are the Precambrian Roraima sandstone formations on the Guiana Shield and the Voltaian sandstone formations in Western Africa. Major occurrences of consolidated sands are found in Northern Africa, in Guyana and Surinam, eastern Peru, northeastern Brazil and in Liberia (western Africa). These sandstone formations have a history of tropical weathering in common they all have a deep weathering mantle of bleached, white sands that are very rich in quartz, poor in clay and excessively drained. Electrolyte contents differ by region In arid and semi-arid areas where evaporation exceeds precipitation, salts and carbonates may accumulate at or near the surface of the soil. 

Higher quartz contents in soils occur in areas where the soil parent materials include quartz-rich materials, such as sandstone bedrock or eolian deposits (area 3 Fig. 2) and in regions where high precipitation accelerates soil weathering and leaching, resulting in high contents of residual, inert quartz (areas 9 and 10 Fig. 2). In Fig. 2 the distribution of Al is nearly... 

Based on predicted weathering and erosion rates of the region, we estimate the profile to be several million years old. Because the soil has developed in situ, the topmost grains have reacted with water for the greatest extent of time. With depth, the total "lifetime" of the particles as soil decreases. This implies that the topmost quartz surfaces should be "reactively mature" (all fines removed, deep grown-together etch pits) and the bottom-most quartz surfaces should be "reactively young" (plentiful fines, fresh surfaces). ... 

Under conditions of intense weathering, silica is unstable in the crystalline form. Mature bauxites, soils representing the most intense weathering conditions, contain no quartz and little combined silica. 

The feldspars are aluminosilicates in which up to half the Si44 ions have been replaced by Al3+ ions. They are the most abundant silicate materials on Earth and are a major component of granite, a compressed mixture of mica, quartz, and feldspar (Fig. 14.45). When some of the cations between the crystal layers are washed away as these rocks weather, the structure crumbles to clay, one of the main inorganic components of soil. A typical feldspar has the formula KAlSi3Og. Its weathering by carbon dioxide and water can be described by the equation... 

In Table 3, susceptibility to weathering increases down the list as fewer silicon-oxygen bonds need to be broken to release silicate. Consequently, quartz and feldspars especially, but also mica in temperate soils, are common inherited minerals in the coarse particle size fractions of soil (the silt and sand fractions, 0.002-2 mm). The amphiboles, pyroxenes, and olivine are much more easily weathered. Thus, soils derived from parent material with rock containing a predominance of framework silicates e.g. granite, sandstone) tend to be more sandy, while those derived from rocks containing the more easily weathered minerals tend to be more clayey. 

In spite of the above issues, consistent trends of increasing surface area with increasing intensity of natural weathering are observed (Brantley et al., 1999). An example of this increase is shown in Figure 12(a) in which BET surfaces of primary minerals increase with increasing age of soils in the Merced chronosequence (White et al., 1996). The extent of this increase depends on the specific mineral phase. The more readily weathered aluminosilicates exhibit greater surface area increases than quartz. Application of BET measurements to characterize fractures and porosity in consolidated rocks has remained generally untested these surface area estimates have been... 

At places in the Amazon the ferrasol is transformed over time to a podzol, a highly weathered, bleached, sandy soil capped by a thick, peat-like accumulation of litter on the top. Everything except quartz is rapidly leached out of the podzol soil including aluminum and iron. The latter are present initially as constituents of organic complexes, but they are not immobilized, because there is a relative lack of clay minerals for adsorption. 

Geochemical processes associated with weathering and soil formation are dominated by alteration of feldspars (and volcanic glass), feldspars accounting for 70% of the upper crust, if the relatively inert quartz is discounted (Taylor and McLennan, 1985). Advancing weathering leads to a shift towards an aluminum rich composition that can be approximated by the chemical index of alteration (CIA) of Nesbitt and Young (1984)... 

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