Calcite decomposition pressures

For the calcite decomposition reaction, the equilibrium constant. Kg, has the same definition as the distribution coefficient, K, given in equation (5.7) giving the equilibrium partial pressure of CO2. The dependence of the equilibrium constant on temperature is given by the Clausius—Claperon equation [2] ... 

That is, since dolomite decomposition was unaffected by the presence of C02 and calcite decomposition can be prevented by a sufficient CO2 over-pressure, Equation (4) was expected to prevail if ankerite decomposition was carried out at low temperatures (< 900 K) in a CO2 environment. Figure 4 shows the results for ankerite decomposition carried out at two different temperatures with and without CC>2 Note that the presence of CO2 completely prevents decomposition at 853 K and severely inhibits decomposition at 900 K. 

Reversible Calcite Decomposition. The reversible nature of calcite decomposition was studied in both the PCM and C-a samples. In the former this was achieved by carrying out the decomposition at C02 pressures less than the equilibrium values. In the latter, the calcite was decomposed to completion in a C02 free atmosphere and then recarbonated at various C02 pressures and temperatures. The data obtained with the PCM sample were fit to the expression shown in Equation (5),... 

Determining equilibrium decomposition pressures of calcite has proved a durable problem, and dubious values have appeared recently... 

The decomposition of dolomite shows many points of similarity with the reactions of calcite and of other single carbonates of Group IIA metals (Sects. 3.1.1 and 3.1.2) the reaction is reversible, occurs at an interface, and both apparent kinetic parameters and reactivity are influenced by the prevailing C02 pressure. 

The nucleation of these decomposition processes was studied by means of thermomicroscopy on single crystal cleavage plates of calcite, magnesite, dolomite and smithsonite (Fig. 59). The shape of the nuclei was found to be different for these carbonates, which might be also of importance for the decomposition mechanism. The partial pressure of water vapor has a pronounced effect on the decomposition of transition metal carbonates such as ZnC03 and CdC03. The evolution of C02 is probably catalyzed in the presence of water vapor and shifted to considerably... 

Such a relation was also found to hold for both cylindrical pellets of calcite and powdered CaC03.21 Cremer and Nitsch,22 in studying the decomposition of CaC03, found that for samples which followed a 2/3 power rate law the pressure dependence of the rate (in m1Bt1) was given by... 

The dissociation pressure of calcite reaches 0.101 kPa (1 atm) at 894°C (S20) and the decarbonation reaction is highly endothermic (Section 3.1.4). The rate of decarbonation becomes significant at 500-600°C if a sufficiently low partial pressure of COj is maintained or if the calcite is intimately mixed with materials, such as quartz or clay mineral decomposition products, that react with the calcium oxide. Even in a precalciner, such mixing occurs, aided by agglomeration caused by the presence of low-temperature sulphate melts. 

Figure 1.11. TEM images of the calcite single crystal shocked in a laser irradiation experiment with an initial pressure of 225 GPa. (a) Dark-field TEM image of a 60-gm-deep zone in the specimen, containing numerous dislocation loops. This microstructurc indicates incipient decomposition, (b) Dark-field TEM image of tangled, curved dislocations occurring in a depth of 90 pm below the initial specimen surface, (c) Bright-field TEM image of a multiply twinned zone in a depth of 280 pm.

Table 9.3 Model calculation applying the computer program PHREEQC (Parkhurst 1995) to a sample of ocean water in a water depth of approximately 1000 m, near the equator. The constant of the solubility product for calcite is corrected for temperature and pressure. The decomposition of organic substance due to the presence of free oxygen in the water column has been included.

The carbon dioxide dissociation pressure for calcite is shown in Table 3.2. At approximately 900 °C, the pressure reaches 1 atmosphere [3.11] — this is generally referred to as the decomposition temperature of calcium carbonate. The decompositions of dolomite and dolomitic limestones are complex and are described more fully in section 15.2. 

White powder or rhombohedral leaflets. Sparingly soluble in HgO, soluble in acid. Vapor pressure at decomposition (321 °C) 77 mm., (357 °C) 760 mm. d 4.258. Crystal structure tjqieGOi (calcite). 

Introduction to the Problem Specific features of the decomposition of carbonates of alkaline-earth metals in the presence of CO2 have been studied over the last 70 years in many works. However, no agreement in quantitative and even qualitative interpretation of these features has been achieved [9]. As an illustration. Table 5.5 presents the reported values of the E parameter for decomposition of calcite in the presence of CO2. As is evident from these data, according to the majority of the studies [10-14], the increase of the CO2 pressure was accompanied by an increase of the E parameter, and in some cases, its magnitude reached 2,000-4,000 kJ moP. By contrast, in other studies [15-17] the E parameter remained approximately constant, although more than a twofold difference in its absolute value was observed. 

If natural calcite crystals are used, then a decomposition curve of the type shown in Fig. 8-13 is observed. The details of the curve will depend upon the deviation of the CO2 partial pressure from the equilibrium value. Such a decomposition curve with a reproducible incubation period indicates that the rate is controlled by the nucleation and growth of oxide particles. 

Marble is a type of recrystallized limestone. The recrystallization process probably occurred about 2000 million years ago in connection with volcanic activity. The carbonates were heated to a high temperature and simultaneously at such a high pressure that decomposition was avoided. Under these circumstances calcite and dolomite were recrystallized to marble. Marble is a fine-grained rock, although the grains may be very small, almost invisible, as in statuario, the completely white Carrara marble. The white semi-transparent Greek marble from the island of Paros was the material for many famous statues. 

Next, low-temperature (<150°C), low-pressure torr) oxygen plasmas oxidized organic components of the sample to CO2. Decomposition of inorganic carbon present (dolomitic limestone rock and calcite/calcium oxalate accretions) was prevented by running the plasmas at low-temperature. Carbon dioxide from the sample was flame-sealed into a glass tube cooled to liquid nitrogen temperature (-194°C), after water had been frozen out with a dry-ice/ethanol slurry (-58°C), and finally sent for radiocarbon analysis at the Center for Accelerator Mass Spectrometry at the Lawrence Livermore National Laboratory (LLNL-CAMS). It was necessary to utilize an AMS measurement due to the small sample size. 

The dehydration curve can be divided in three steps that successively correspond to the evolution of 1) the zeolitic and adsorbed water on external surface (T < 7S°C), 2) coordination water linked at the edge magnesium atoms inside of the channels (in two times, domains 75-150°C and 150-370°C, two molecules each and weight losses 10.48% of the final mass), 3) structural (2.61 %), one molecule due to two hydroxyls from the octahedral layer of the talc ribbon) and decomposition of calcite. According to ref. 3, the structure folds when approximately half of the coordination water is removed, i.e. here under 2 Pa residual pressure between 100 and 130 C. 

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