Silicates weathering

In Figure 9.21 all of the carbon eventually used in weathering of minerals by CC>2-charged soil water is shown as entering the atmosphere. The difference between the flux of CO2 owing to precipitation of carbonate minerals in the ocean and the total CO2 released from the ocean is that CO2 used to weather silicate minerals on land, and agrees with the calculations of riverine source materials made earlier in this chapter, in which it was shown that 30% of the HCC>3 in river water comes from weathering of silicate minerals. 

We have seen above that the kinetics of mineral dissolution is well explained by transition-state theory. The framework of this theory and kinetic data for minerals have shown that dissolution is initiated by the adsorption of reactants at active sites. Until now these active sites have been poorly characterized nevertheless, there is a general consensus that the most active sites consist of dislocations, edges, point defects, kinks, twin boundaries, and all positions characterized by an excess surface energy. Also these concepts have been strongly supported by the results of many SEM observations which have shown that the formation of crystallographically controlled etch pits is a ubiquitous feature of weathered silicates. 

Container Insulation Tanks containing materials above atmospheric temperature may require insulation to reduce loss of heat. Almost any of the commonly used insulating materials can be employed. Calcium silicate, glass fiber, mineral wool, cellular glass, and plastic foams are among those used. Tanks exposed to weather must have jackets or protective coatings, usually asphalt, to keep water out of the insulation. 

Water hase (sodium or potassium silicate) Tough ahrasion-resistant excellent chemical hond Good Poor Poor Excellent Excellent Excellent unaffected hy weather Easy... 

That these elements have in the past been considered unfamiliar has been due largely to the difficulties involved in preparing the pure metals and also to their rather diffuse occurrence. Like their predecessors in Group 3, they are classified as type-a metals and are found as silicates and oxides in many silicaceous materials. These are frequently resistant to weathering and so often accumulate in beach deposits which can be profitably exploited. 

Late rites, which are oxide/silicate ores such as gamierite, (Ni,Mg)6Si40io(OH)8, and nickeliferous limonite, (Fe,Ni)0 (0H).nH20, which have been concentrated by weathering in tropical rainbelt areas such as New Caledonia, Cuba and Queensland. 

Zinc (76ppm of the earth s crust) is about as abundant as rubidium (78 ppm) and slightly more abundant than copper (68 ppm). Cadmium (0.16 ppm) is similar to antimony (0.2 ppm) it is twice as abundant as mercury (0.08 ppm), which is itself as abundant as silver (0.08 ppm) and close to selenium (0.05 ppm). These elements are chalcophiles (p. 648) and so, in the reducing atmosphere prevailing when the earth s crust solidified, they separated out in the sulfide phase, and their most important ores are therefore sulfides. Subsequently, as rocks were weathered, zinc was leached out to be precipitated as carbonate, silicate or phosphate. 

Dissolution of CaCOs is a congruent reaction the entire mineral is weathered and results completely in soluble products. The above reaction is driven to the right by an increase of CO2 partial pressure and by the removal of the Ca and/or bicarbonate. Any impurities present in the calcareous rock, such as silicates, oxides, organic compounds, and others, are left as residue. As the calcium and bicarbonate leach... 

A specific example of this would be the weathering of K-feldspar and the formation of kao-linite (see Table 8-1 for mineral definitions), a layer-silicate clay ... 

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. 

Once a layer-silicate clay forms, it does not necessarily remain in the soil forever. As conditions change it too may weather and a new mineral may form that is more in equilibrium with the new conditions. For example, it is common in young soils for the concentrations of cations such as K, Ca, or Mg in the soil solution to be high, but as primary minerals are weathered and disappear, cation concentrations will decrease. With a decrease in solution cations, a layer-silicate such as vermiculite will no longer be stable and can weather. In its place. 

Blum, A. E. and Stillings, L. L. (1995). Feldspar dissolution kinetics. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. 

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