Dolomites stability

Fig. 24. Present-day dolomite stability trend with depth in the Barracouta field. AP Activity product K equilibrium constant. A, B, C, D represent the depths of the layers modeled in Fig. 23...

Dead-burned dolomite is a specially sintered or double-burned form of dolomitic quicklime which is further stabilized by the addition of iron oxides. Historically, it was used as a refractory for lining steel furnaces, particularly open hearths, but as of this writing is used primarily in making dolomite refractory brick (see Refractories). 

PefractoTy lime is synonymous with dead-burned dolomite, an unreactive dolomitic quicklime, stabilized with iron oxides, that is used primarily for lining refractories of steel furnaces, particularly open hearths. 

Stability. AH calcitic and dolomitic limestones are extremely stable compounds, decomposing only in fairly concentrated strong acids or at calcining temperatures of 898°C for high calcium and about 725°C for dolomitic stones at 101.3 kPa (1 atm). A very mild destabilizing effect is caused by C02-saturated water, as described in the preceding section on solubihty. Aragonite, however, is not as stable as calcite. In sustained contact with moisture,... 

Even higher temperatures are required for calcite dissociation. As f>co2 is increased to 760 Torr, the reaction temperature rises to 1170 K and the extent of dissociation is diminished [29]. The rate of decomposition of dolomite in vacuum [734] was intermediate between those for magnesite and calcite. Ranges of study were magnesite 810—870 K, dolomite 910— 990 K, and calcite 990—1050 K. Values of E were in the different sequence, magnesite < calcite < dolomite. Magnesite, which would decompose very rapidly at the temperature of dolomite dissociation, is, therefore, relatively stabilized, whereas the reactivity of calcite is enhanced in the mixed crystal. 

Chemical stabilizers have been used to reduce the rate of oxygen-promoted degradation of polysaccharides at T>225°F. Methanol and sodium thiosulfate are the most commonly used (86). Sodium dithio-carbamate, alkanolamines, and thiol derivatives of imidazolines, thiazolines, and other heterocyclic compounds have also been tested for this application. Calcined dolomite (B7) and Cu(l) and Cu(ll) salts (88) have been reported to increase the thermal stability of HEC. 

Carpenter, A. B., 1980, The chemistry of dolomite formation I the stability of dolomite. Society of Economic Paleontologists and Mineralogists Special Publication28,111-121. 

Lafon, G. M., G. A. Otten and A. M. Bishop, 1992, Experimental determination of the calcite-dolomite equilibrium below 200 °C revised stabilities for dolomite and magnesite support near-equilibrium dolomitization models. Geological Society of America Abstracts with Programs 24, A210-A211. 

A general problem with CaO, limestone and dolomite is the limited lifetime of the C02 acceptor material [32], The capacity for C02 is initially very high, but is depleted to almost zero after several cycles. Although the minerals are relatively cheap, this would imply a very considerable stream of waste material coming out of the hydrogen plant. Novel materials are in development with a higher stability [39]. 

CO, SC ), or the occurrence of the minerals is sufficiently rare to represent a special case—the various halide salts, for example. However, dolomite presents a special problem in that the existence of Mg is important to silicate equilibria under consideration. The main trouble here is that the conditions of crystallization and stability of dolomite in sediments and sedimentary rocks is imperfectly known, thus leaving a question as to its influence on silicates or the influence of silicates on its presence. One is forced more or less to ignore the importance of dolomite at present. This does not mean that it can be ultimately excluded from a complete discussion of clay mineral stability. 

M. Gregor et al (Czech), IndChimBelge 1967, 32 (Spec No,Pt2), 373-76 CA 71, 3l9l6y(l969) (Stabilization of AN with dolomite has advantage over stabilization by... 

Because free gas (or gas-saturated water) is less dense than either water or sediments, it will percolate upward into the region of hydrate stability. Kvenvolden suggested that a minimum residual methane concentration of 10 mL/L of wet sediment was necessary for hydrate formation. The upward gas motion may be sealed by a relatively impermeable layer of sediment, such as an upper dolomite layer (Finley and Krason, 1986a) or the upper siltstone sequence, as in the North Slope of Alaska (Collett et al., 1988). Alternatively, permafrost or hydrate itself may act as an upper gas seal. These seals can also provide traps for free gas that has exsolved from solution, and the seals can subsequently act to provide sites for hydrate formation from the free gas. 

Several water compositions are plotted on the stability diagrams in Figure 8.2. It can be seen that at shallow Earth surface pressures and temperatures, seawater plots in the stability field of dolomite whereas solutions of average river water composition and most shallow groundwaters plot in the field of calcite. With burial of carbonate sediments and elevated P and T, the dolomite field shrinks, but subsurface fluid compositions evolve toward a composition in equilibrium with dolomite. This conclusion is probably one of the most important arguments for the formation of dolomite during deep burial diagenesis (see also Hardie, 1987). Thermodynamic considerations favor this reaction path, as well as the fact that... 

Figure 8.13. Plot of expected stability field for ordered dolomite as a function of temperature, and Ca2+ Mg2+ ion ratio in 1 m and 2 m chloride brines. At the top is a histogram of Ca2+ Mg2+ ion ratios of groundwaters in contact with a variety of sedimentary rocks, showing that at temperatures above 60-70°C many Ca-rich groundwaters could be dolomitizing fluids. (After Hardie, 1987.)...

Carbonate has been observed infrequently as inclusions in diamond. Calcite was described by Meyer and McCallum (1986) and Brenker et al. (2002), magnesite by A. Wang et al. (1996), and dolomite by Stachel et al. (1998). Of the three, the magnesite is least surprising, in that magnesite is the carbonate that is stable in peridotitic assemblages in the diamond stability field. 

The dolomite observed by Stachel et al. (1998) was a single-phase inclusion. The presence of dolomite within the stability field of diamond requires a protolith in which the exchange reaction Mg2Si20s + CaMg(C03)2 = CaMgSi20g + 2 MgCOs is prevented from occurring by the absence of orthopyroxene. 

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