Carbonate minerals, weathering

It has been shown (20,21) that siUcate weathering is more important than carbonate mineral weathering as a longterm control on atmospheric CO2. The HCO3 and ions produced by weathering of CaCO precipitate in the... 

Sulfate, halide, and carbonate minerals form in mine waste as a result of chemical weathering reactions and as a by-product of mineral processing. The formation of carbonate minerals is of particular interest for its potential in offsetting greenhouse gas emissions associated with mining. We have documented secondary carbonate mineral precipitation at the Mount Keith Nickel Mine (Western Australia) and the... 

Pyrite is not the only Fe2+-bearing mineral that undergoes oxidative weathering when coalbearing strata are exposed to the atmosphere through mining. The Fe-rich carbonate minerals... 

The over-all picture of what happens to the soil waters, as illustrated by Tables II and IV, is that initially they rapidly attack the rocks, kaolinizing chiefly plagioclase plus biotite and K-spar. As they penetrate more deeply, the reaction rate slows down, and both kaolinite and mont-morillonite are weathering products. Also, an important part of the Ca2+ comes from solution of small amounts of carbonate minerals. 

Fig. 5.10. Conceptual model of carbonic acid weathering of ferromagnesian minerals on Mars. Reprinted from Marion et al. (2003a) with permission...

This process consumes oxygen and produces C02. As a result, the oxygen content of air in soil may be as low as 15%, and the carbon dioxide content may be several percent. Thus, the decay of organic matter in soil increases the equilibrium level of dissolved C02 in groundwater. This lowers the pH and contributes to weathering of carbonate minerals, particularly calcium carbonate. 

In the previous section the present-day cycle of carbon was discussed in some detail. We can now turn our attention to the sources of carbon to the ocean, and of calcium and magnesium, the major elements (other than oxygen and hydrogen) with which carbon interacts. Because carbon dioxide is the major acid gas involved in both carbonate and silicate mineral weathering reactions at the Earth s surface, it is informative to consider the sources of other elements as well. 

Notice that in step 2, the weathering of an aluminosilicate mineral by the aggressive attack of "H2S04" waters may result in the release of dissolved silica, as well as cations. The ratio of silica to cations released depends on the composition of the primary mineral weathered and its alteration product. The weathering of a carbonate mineral gives cations and bicarbonate. 

This is a tentative balance because of a number of poorly known estimates, but it does illustrate the link between sulfide mineral weathering and resultant acid attack on carbonate and silicate minerals. 

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. 

Therefore, as an oxide mineral weathers, reactions of C02 and water at the surface lead to the formation of carbonates and bicarbonates. The presence of OH can eventually cause... 

Among the most common minerals are the feldspars and clays. These materials have been used for centuries in the manufacture of pottery, china, brick, cement, and other materials. Feldspars include the mineral orthoclase, K20 A1203 6Si02, but this formula can also be written as K2Al2Si60i6- Under the influence of carbon dioxide and water, this mineral weathers by a reaction that can be shown as... 

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