Calcite composition

To set up the calculation, we specify initial isotopic compositions for the fluid and calcite. We choose a value of —13 %c for r) 18 Os mow of the Lyons fluid, reflecting Tertiary rainfall in the region, and set the calcite composition to +11 %o, the mean of the measured values (Fig. 25.4). We further set S13C pi)B for the fluid to — 12 %o. We do not specify an initial carbon composition for the calcite, so the model sets this value to — 11 %c, in isotopic equilibrium with the fluid. Again, this value is near the mean of the measurements. 

Figure 5.10A illustrates the overall pattern of skeletal magnesian calcite compositions from the tropics to the poles. Data from major organism groups are plotted from Chave (1954a). This trend of compositional change with latitude may be interpreted as reflecting the influence of ... 

Carbonate rock fragments could also be a source of calcite cement if they are available in the source area and if they survive transport. In the Palaeogene sandstones of the North Sea we have found intervals containing up to 55 vol% chalk clasts, for instance, and carbonate rock fragments have also been reported from sandstones in other basins (e.g. Richmann et al., 1980 Dickinson, 1988 Taylor, 1990). However, carbonate rock fragments are probably more stable than biogenic carbonate because of their low-Mg-calcite composition and coarser crystal size. 

The relative dissolution rates between calcite and dolomite were also estimated for Bonanza field by examining the ratio of excess Ca to excess Mg in the post-treatment produced water. This ratio is 2.6 (Fig. 8). Assuming pure calcite (CaCOj) and pure dolomite (CaMg(C03)2>, this could only have resulted from the dissolution of 1.6 mol of calcite and 1 mol of dolomite, a calcite/dolomite dissolution ratio of 1.6. Assuming an impure calcite composition, the Ca/Mg ratio would result in a calcite/ dolomite dissolution ratio of approximately 1.9. Even though calcite is a minor component in the Tensleep reservoir, its relative contribution to the post-COj injection water is great. This is also despite the fact that the dolomite is more finely crystalline than the calcite, and would thus be expected to have a larger surface area exposed to the carbonated water. 

Stearic-acid-treated calcite powder is used in the production of PVC pipes (because of its low affinity for humidity), of soft or rigid plastic shapes, and of other composites. Calcite is also used in composites designed for electrical insulators and cables. In the case of cross-linked PE, calcite improves its dielectric properties, toughness, ductility, elasticity, and production cost. Calcite powder specially treated for PP and thermoplasts is available from the following producers Pfizer Inc., Thompson, Weinman and Co., and Georgia Marble (U.S.). PP-calcite composites contain up to 40% of calcite. 

---