Carbonate minerals compositions

Figure 2.4. Carbonate mineral compositions in the system CaC03-MgC03-FeC03 - MnCC>3 for the lower (A) and the upper (B) portions of the Marra Mamba Iron-formation. The apices of the triangle represent 100 mole % of the indicated component. The tie-lines connect possible coexisting phases (i.e physically touching pairs of carbonate minerals). The numbers I-III on the tie lines represent three different possible configurations. Data from Klein and Gole (1981), as represented in Woods (1988).

Natural carbonate minerals do not form from pure solutions where the only components are water, calcium, and the carbonic acid system species. Because of the general phenomenon known as coprecipitation, at least trace amounts of all components present in the solution from which a carbonate mineral forms can be incorporated into the solid. Natural carbonates contain such coprecipitates in concentrations ranging from trace (e.g., heavy metals), to minor (e.g., Sr), to major (e.g., Mg). When the concentration of the coprecipitate reaches major (>1%) concentrations, it can significantly alter the chemical properties of the carbonate mineral, such as its solubility. The most important example of this mineral property in marine sediments is the magnesian calcites, which commonly contain in excess of 12 mole % Mg. The fact that natural carbonate minerals contain coprecipitates whose concentrations reflect the composition of the solution and conditions, such as temperature, under which their formation took place, means that there is potentially a large amount of information which can be obtained from the study of carbonate mineral composition. This type of information allied with stable isotope ratio data, which are influenced by many of the same environmental factors, has become a major area of study in carbonate geochemistry. 

Solid solution theory The chemical theories of primary importance to understanding factors controlling carbonate mineral compositions in natural systems are associated with solid solutions. Carbonate minerals of less than pure composition can be viewed as mixtures of component minerals (e.g., SrCC>3 and CaSC>4 in CaCC>3). If the mixtures are of a simple mechanical type then the free energy of formation of the resulting solid will be directly proportional to the composition of the aggregate. Thus, for a two component, a and b, mixture ... 

In addition to magnesium, at least trace amounts of many components present in seawater can be incorporated into marine carbonates. Concentrations range from trace (e.g., heavy metals), to minor (e.g., strontium), to major (e.g., magnesium, see previous discussion). This means that there is potentially a large amount of information that can be obtained from the study of carbonate mineral composition. This type of... 

The carbonate minerals in these sandstones are especially interesting in this regard because it is possible to compare their compositions with those of similar phases of documented Alleghanian association in the thrust Lower Palaeozoic carbonate units to the southeast. Marked differences between carbonate mineral compositions in the relatively undeformed foreland (reported here) and those in the fold and thrust belt (reported in the literature), suggest constraints on models for fluid flow during Alleghanian orogenesis. 

Solid rock type (sandstone or carbonate), mineral composition (clays, other minor or trace minerals), wettability, surface charge, and specific surface area... 

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