CaCC>3 calcite

SAQ 3.10 Calcite and aragonite are both forms of calcium carbonate, CaCC>3. Calcite converts to form aragonite. If A"f°(calcite) = -1206.92 kJ... 

Comparison of the chemical composition of rivers Calancasca (southern Switzerland) draining silicate rocks and Glatt (northern Switzerland) draining CaCC>3 (calcite) bearing terrain. Because of the high population density anthropogenic effects are superimposed on the geological effects. 

Fig. 14.15 X -ray diffraction patterns of the CaCC>3/halloysite composite after CaC03 formation. The first peak at 12.3° corresponds to 7.2 A (dehydrated halloysite packing reflection). Arrows indicate peaks corresponding to the vaterite modification of CaC03. Calcite modification of CaC03 was not found. Peak assignment was made using the Powder Diffraction Database.

In a first example of how minerals can buffer a fluid s chemistry, we consider how a hypothetical groundwater that is initially in equilibrium with calcite (CaCC F) at 25 °C might respond to the addition of an acid. In REACT, we enter the commands... 

Equilibrium composition of a solution in presence of CaCC>3(s) (calcite) at constant particle pressure of C02 (Pco2 = 10 3 5 atm) at 25° c-,f no ac d or base is added-the equilibrium composition is indicated by the arrow. The calcite at this composition is characterized by a net surface charge of zero. 

Here are some standard thermo chemical data relevant to lime burning (limestone can be taken to be pure, solid CaCC>3 in its calcite form) ... 

Polymorph Two or more solid crystalline substances with the same chemistry, but different crystalline structures. Calcite (rhombohedral CaCCE) and aragonite (orthorhombic CaCCE) are polymorphs. 

Calculate the number of grams in a mole of each of the following common substances (a) calcite, CaCC>3 ... 

Figure 2.1. Generalized phase relations in the unary system CaCC>3. "A" -aragonite "I" through "V - calcite polymorphs with metastable fields indicated by (). Dash-dot line at 800°C represents transition encountered on cooling runs solid line at lower T represents transition encountered on heating runs. (After Carlson, 1980.)...

Holland H.D., Holland H.J. and Munoz J.L. (1964) The coprecipitation of cations with CaCC>3. II. The coprecipitation of Sr2+ with calcite between 90 and 100°C. Geochim. Cosmochim. Acta 28,1287-1302. 

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