Dolomites compositions

For each CO2 fugacity chosen, the dolomite composition during the reaction approaches a steady state, as can be seen in Figure 19.1. The steady state reflects the... 

Figure 8.24. Summary of composition of nonstoichiometric dolomite. A. Data for 246 samples from continental exposures of dolostones and dolomitic limestones. B. Data for 153 samples from continental exposures of dolomites related and not related to evaporite sequences. C. Ranges of Holocene dolomite compositions. D. Data for 35 samples of deep-marine dolomite from DSDP cores. E. Histogram of 245 samples of deep-marine dolomite E evaporite-associated, O organic origin, L samples of Lumsden (1988) above, D detrital, and U uncertain. (After Lumsden, 1988.)...

Another similarity between Monterey and Stevens dolomites is their excess Ca, typical of Tertiary dolomite compositions. Apparently these non-stoichiometric compositions are retained in San Joaquin dolomites at temperatures at least as high as 100°C (their present burial condition). Stevens dolomites are generally much richer in Fe than Monterey dolomites, possibly owing to the alteration of Fe-bearing minerals in the Stevens sandstones. 

Quantitative determination of elemental dolomite composition was carried out on five polished thin sections covered with a thin layer of carbon and using a CAMECA SX 51 electron microprobe at 15 kV, a 20 nA beam current and a 0.2 pm beam diameter. The BSE imaging system linked to the electron microprobe was used to detect zonation in the dolomite cement, and compositional analyses were carried out for each zone. Results were normalized to 100 mol% Fe, Mg and Ca. The precision of the analyses was 100% 2. 

Fig. 11. Ternary plot of ferroan dolomite compositions measured by electron probe microanalysis.

Fig. 3. Compilation of iron and manganese concentrations in saddle dolomite (note logarithmic scale). Same key as in Fig. 2. The dashed line marks the boundary between Fe-dolomite and ankerite according to Deer et al. (1992). Some studies provide Fe and Mn concentrations (reported as wt% or ppm) but lack Ca and Mg data. To achieve consistent presentation, Fe and Mn values were recalculated as mol% (Fe, Mn)C03, assuming a dolomite composition of 50 mol% CaC03 (marked by an asterisk). Excess Ca results in systematically low (Fe,Mn)C03 values, but the error is negligible. Data from Wojcik et al. (1992) on saddle dolomite and ankerite are plotted in the limestone and sandstone categories, rather than the mixed category (see Table 1). Data published by Searl Fallick (1990) could not be included because of ambiguous data identification.

Quicklime and hydrated lime are reasonably stable compounds but not nearly as stable as their limestone antecedents. Chemically, quicklime is stable at any temperature, but it is extremely vulnerable to moisture. Even moisture in the air produces a destabilizing effect by air-slaking it into a hydrate. As a result, an active high calcium quicklime is a strong desiccant (qv). Probably hydrate is more stable than quicklime. Certainly hydrated lime is less perishable chemically because water does not alter its chemical composition. However, its strong affinity for carbon dioxide causes recarbonation. Dolomitic quicklime is less sensitive to slaking than high calcium quicklime, and dead-burned forms are completely stable under moisture-saturated conditions. 

Chry sotile is a hydrated magnesium siHcate and its stoicliiometric chemical composition may be given as AIg2Si20 (0H)4 [12001 -29-5]. However, the geothermal processes wliich ield the chry sotile fiber formations usually involve the co-deposition of v arious other minerals. Tliese mineral contaminants comprise brucite [1317-43-7] (AIg(OH)2), magnetite [1309-38-2] (Fe O, calcite [13397-26-7] (CaCO ), dolomite [16389-88-1] (AIg,CaC02),... 

Thermal analysis has been widely and usefully applied in the solution of technical problems concerned with the commercial exploitation of natural dolomite including, for example, the composition of material in different deposits, the influence of impurities on calcination temperatures, etc. This approach is not, however, suitable for the reliable determination of kinetic parameters for a reversible reaction (Chap. 3, Sect. 6). 

Figure 1.31. Bivariable plot of oxygen versus carbon isotopic compositions of carbonates. Solid circle magnesite open circle dolomite open square calcite A oxygen and carbon isotopic compositions of igneous carbonates B oxygen and carbon isotopic compositions of marine carbonates (Shikazono et al., 1995).

In our example, we test the consequences of reacting an isotopically light (i.e., nonmarine) limestone at 60 °C with an isotopically heavier groundwater that is relatively rich in magnesium. We start by defining the composition of a hypothetical groundwater that is of known CO2 fugacity (we initially set /co2 to 1) and in equilibrium with dolomite ... 

Fig. 19.2. Isotopic composition (bold lines) of dolomite formed by reaction between a limestone and migrating groundwater, assuming that minerals maintain isotopic equilibrium over the simulation. Fine lines show results of simulation holding minerals segregated from isotopic exchange, as already presented (Fig. 19.1).

Since we have no direct information about the chemistry of the Fountain fluid, we assume that its composition reflects reaction with minerals in the evaporite strata that lie beneath the Lyons. We take this fluid to be a three molal NaCl solution that has equilibrated with dolomite, anhydrite, magnesite (MgCC>3), and quartz. The choice of NaCl concentration reflects the upper correlation limit of the B-dot (modified Debye-Hiickel) equations (see Chapter 8). To set pH, we assume a CO2 fugacity of 50, which we will show leads to a reasonable interpretation of the isotopic composition of the dolomite cement. 

We can predict the oxygen and carbon isotopic compositions of the dolomite produced by this reaction path, using the techniques described in Chapter 19. Figure 25.4 shows the compositions of calcite and dolomite cements in the Lyons, as determined by Levandowski et al. (1973). The calcite and dolomite show broad ranges in oxygen isotopic content. The dolomite, however, spans a much narrower range in carbon isotopic composition than does the calcite. 

Fig. 25.4. Oxygen and carbon stable isotopic compositions of calcite ( ) and dolomite ( ) cements from Lyons sandstone (Levandowski et al., 1973), and isotopic trends (bold arrows) predicted for dolomite cements produced by the mixing reaction shown in Figure 25.3, assuming differing CO2 fugacities (25, 50, and 100) for the Fountain brine. Fine arrows, for comparison, show isotopic trends predicted in calculations which assume (improperly) that fluid and minerals maintain isotopic equilibrium over the course of the simulation. Figure after Lee and Bethke (1996).

Fig. 30.1. Volumes of minerals precipitated during a reaction model simulating the mixing at reservoir temperature of seawater into formation fluids from the Miller, Forties, and Amethyst oil fields in the North Sea. The reservoir temperatures and compositions of the formation fluids are given in Table 30.1. The initial extent of the system in each case is 1 kg of solvent water. Not shown for the Amethyst results are small volumes of strontianite, barite, and dolomite that form during mixing.

The beneficiation of mixed lead zinc sulphide oxide ores is a complex process and is dependent on gangue composition of the ore. There are two basic types of mixed sulphide oxide ores that have been extensively studied. These include (a) ores with dolomitic and... 

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