SUBJECTS calcite

Figure 7.42. Comparison between (A) an idealized plot of variation in 8180 and 813C for carbonates subjected to vadose and phreatic meteoric diagenesis (after Lohmann, 1988) with (B) the meteoric alteration trend observed for the Key Largo Limestone, Florida, U.S.A. (after Martin et al., 1986). The critical trend in isotopic composition is termed the meteoric calcite line. This trend may be modified at the water recharge surface where evaporation is an important process, caliche is formed and the diagenetic phases are depleted in 13C derived from soil-gas CO2. Another modification can occur distally to the recharge area where precipitating carbonate cements may have isotopic ratios nearly equivalent to dissolving phases.

This paper reviews the subject of the kinetics of calcite dissolution and precipitation by comparing predictions made by our mechanistic model with published laboratory results. 

Example 7.9. Effect of Pressure and Temperature on Calcite Solubility in Seawater An enclosed sample of the surface seawater, as discussed in Example 7.8 (25°C pH = 8.2 [Ca ] = 1.06 x 10" M [Carb-Alk] = 2.4 x 10" eq liter" ), is cooled to 5°C and then subjected to increases in total pressure of up to 1000 atm (equivalent to exposing the sample to increased water depths of approximately 10,000 m). How does the composition, pH, [CO3"], [Ca "], and extent of oversaturation change as a result of the temperature change at 1 atm and as a result of the pressure change at 5°C The water is incipiently oversaturated with respect to calcite. Assume that CaC03... 

Thomas and Renshaw [7] studied the role of dislocations in nucleus formation and early growth in reaction (12.1). The large calcite crystals used were first subjected... 

Two Colorado oil shale samples one from the Parachute Creek Member and the other from the C-a tract, were retorted, de-charred and then subjected to temperatures between 800 K and 1100 K in order to study the mineral reactions which take place. Comparisions between these two samples include the reversible nature of ankeritic dolomite and free calcite as well as the temperatures at which significant silication takes place. Results for the C-a tract samples indicated silication appears to take place in stages and that ankeritic dolomite decomposition can be prevented by relatively low CO2 concentrations. Ankeritic dolomite and calcite decomposition rates were similar for the two samples and there was strong evidence that calcite recarbonation takes place via non-activated chemisorption of C(>2 ... 

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