Calcite stabilities

Aragonite is decompressed to calcite stability field Magma is cooled to below the liquidus Gas-bearing magma is decompressed to oversaturation... 

Vacher H.L., Bengtsson T.O. and Plummer L.N. (1989) Hydrology of meteoric diagenesis - Ground-water residence times and aragonite-calcite stabilization rates in island fresh-water lenses. Geol. Soc. Amer. Bull, (in press). 

Walter, L.M. Morse, J.W. (1984) Magnesian calcite stabilities a reevaluation. Geochim. Cosmochim. Acta, 48, 1059-1069. 

Figure 2.58 Three-dimensional diagram of thermodynamical stability for CaCOj- Ca MgfCO.T- H O system (Machel, and Mountjoy, 1986 Braithwaite et a/., 2004). The seawater point (SW) is within calcite stability field. The dashed field border is inclined toward higher Ca/Mg ratios at salinity higher than 35%o.

But Equation (4.9) does not apply to irreversible processes in which a metastable state changes to a stable state, such as a crystal of aragonite at some T and P within the calcite stability field recrystallizing to calcite. Equations (4.14) and (4.46) apply to these cases. In these cases a second form of work (non-Py work) is being done. 

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. 

Morse JW, Bender ML (1990) Partition coefficients in calcite Examination of factors irrflnencing the validity of experimental resnlts and their application to natural systems. Chem Geol 82 265-277 Mucci A, Morse JW (1990) The chemistry of low temperature abiotic calcites Experimental studies on coprecipitation, stability and fractionation. Rev Aquatic Sci 3 217-254 Musgrove ML, Barmer JL, Mack LE, Combs DM, James EW, Cheng H, Edwards RL (2001) Geochronology of late Pleistocene to Holocene speleothems from central Texas Implications for regional paleoclimate. Geol Soc Am Bull 113 1532-1543... 

Homopolymers. Polymers such as poly(methacrylamido-4,4,8,8-tetra-methyl-4,8-diaza-6-hydroxynonamethylene dichloride), abbreviated poly(MDTHD), and a triaza analog, abbreviated poly(MTHHDT), have been shown to be effective stabilizers of silica, calcite, and hematite (14,15) as indicated by the data summarized in Table V. 

Copolymers of MDTHD and DMAPMA appeared to be the most effective silica, calcite, and hematite mineral fines stabilizers. Increasing the copolymer MDTHD content had little effect on polymer performance. Similar results were observed for a series of MDTHD -DMAEMA copolymers and a series of DMAEMA CH-C1 salt - DMAEMA copolymers (Table VI). In contrast, increasing the MDTHD content of MDTHD - NNDMAm copolymers from 67% to 90% improved copolymer performance as a silica fines and hematite fines stabilizer. 

Limited silica fines stabilization data indicated that increasing copolymer molecular weight from 100,000 to 1,000,000 daltons had, if anything, a negative effect on silica fines stabilization. At a molecular weight of 1,000,000 daltons, this copolymer appeared to be more effective in stabilizing silica fines than silica/kaolinite, calcite, or hematite fines. However, the results may be due in part to the larger particle size and lower surface area of the silica fines (see Table II). 

When the DMAEMA content of NVP - DMAEMA copolymers was reduced from 20% to 8%, the silica fines stabilization effectiveness appeared to improve slightly. When the 80/20 NVP - DMAEMA copolymer was converted to a terpolymer containing 8% DMAEMA (CH SO, silica fines stabilization was substantially unaffected. However, stabilization of silica/kaolinite fines was greatly improved. This suggested that the interaction of polymer quaternary nitrogen atoms with anionic sites on mineral surfaces was important for the stabilization of migrating clays but a different interaction was important for the stabilization of silica fines. Calcite fines stabilization improved while hematite fines stabilization effectiveness decreased. This also indicated the nature of the adsorbed polymer - fine particle complex varied for different minerals. 

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. 

Bischoff, W. D., F. T. Mackenzie, and F. C. Bishop (1987), "Stabilities of Synthetic Magnesian Calcites in Aqueous Solution Comparison with Biogenic Materials," Geochim. Cosmochim. Acfa51, 1413-1423. 

As described in Chapter 21.7, a system of biogeochemical feedbacks act to stabilize the major ion composition of seawater. Some operate on short time-scale cycles, such as calcite compensation, and others operate over longer periods, such as the basalt-carbonate buffer. The linkages in the crustal-ocean-atmosphere fectory that act on the major ions also influence atmospheric CO2 levels and seawater s pH and alkalinity. 

Much stronger kinetic stabilization can be expected for processes leading to the inclusion of radionuclide ions into the colloid structure (Fig. 7, lower part). Spectroscopic indications for such processes have indeed been found again by TRLFS for the Cm(III) interaction with colloidal and particulate amorphous silica, calcite and CSH phases (Chung et al. 1998 Stumpf Fanghanel 2002 Tits et al. 2003). The incorporation of actinide ions into colloidal precursor clay phases has been recently investigated as a possible mechanism in natural... 

Chapters 15 and 16 especially demonstrate the broad range of application of thermodynamics to chemical processes. In the discussions of the Haber cycle, synthesis of diamond, solubility of calcite, and the thermodynamics of metabolism, techniques are used to solve a specific problem for a particular substance. On the other hand, in the discussion of macrocyclic complexes, the description and interpretation involves the comparison of the properties of a number of complexes. This global approach is particularly helpful in the description of the energetics of ternary oxides in Chapter 15 and the stabilities of proteins and DNA in Chapter 16, where useful conclusions are obtained only after the comparison of a large amount of experimental data. 

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