Dolomitization open system

Sperber et al. (1984) conducted an extensive examination of Paleozoic dolomites from North America that provides general insight into how different types of dolomitization processes may be reflected in carbonate rocks. Their data indicate that two separate processes may lead to two distinctive populations of sedimentary dolomite. The processes are divided into closed and open system... 

Figure 7.13. Conceptual model for the relationship between rock composition, openness of system, and dolomite stoichiometry. Shallow formation of dolomite produces less "ideal" dolomite. More open systems tend to produce more complete dolomitization. (After Sperber et al 1984.)...

Figure 6.14 Idealized, end-member approaches to mineral saturation of groundwater, dissolving calcite or dolomite at 12°C and 1 bar total pressure. Open-system dissolution pathways are labeled with hxed values of PCOj- Closed-system pathways are labeled no CO2 added. Reprinted from Geochim. et Cosmochim. Acta, 35, D. Langmuir, The geochemistry of some carbonate ground waters in central Pennsylvania, 1023-45, 1971, with permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 1GB, U.K.

Land (e.g., 198S) has stressed the chemical and structural variations associated with natural dolomites, and has gone so far as to suggest that the name "dolomite" be used in the same way that the mineral name "feldspar" is used. The fact that dolomite is relatively unreactive compared to most other sedimentary carbonate minerals has severely limited experimental studies under temperature and pressure conditions that exist during shallow burial. Consequently, most information on the chemical behavior of dolomite must be obtained from observations of complex natural systems. Such observations are all too often open to multiple interpretations. 

Based on a knowledge of the mineralogy of the Uinta Sandstone, the mineral phases most likely to be controlling the solubility of Ca in the sandstone - L2 leachate system are calcite, dolomite, gypsum, and fluorite. Stability lines and saturation indices calculated for these minerals are present in Figure 1 and Thble III, respectively. The observed data point (black circle) plotted in Figure 1 represents the measured pH and log Ca + activity in the L2 leachate after reaction with the Uinta Sandstone. The log CO2 gas partial pressure of -2.95 atmosphere is based on the measured pH and alkalinity of the reacted solution. The open circle represents the log Ca + activity and pH calculated by MINTEQ for the raw leachate recarbonated to a log CO2 partial pressure of -2.95 atmosphere. The leachate apparently developed a CO2 gas overpressure because equilibrium with calcite was attained in sealed containers at the relatively low pH of 7.91 (18). The calcite-dolomite line shown in the figure represents the pH-dependent activity of Ca + in equilibrium with both calcite and dolomite. 

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