Magnesite stability

The first two methods are preferred in industrial applications. They are used for producing porous insulating materials of fireclay, silica, high-alumina materials, magnesite, stabilized zirconia etc. Their thermal conductivity is several times lower than that at a standard porosity (cf. Fig. 214). 

Rosenberg, P.E. and Holland, H.D., 1964. Calcite—dolomite—magnesite stability relations in solutions at elevated temperatures. Science, 145 700—701. 

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. 

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. 

Refractory Magnesite, 92% MgO Magnesite—chrome, CBc CBc Fired Forsterite, stabilized Chrome, fired 90% 70%... 

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 Stability of Hydroxides, Carbonates, and Hydroxide Carbonates 395 For magnesite ... 

They are normally cast in the form of brick and are sometimes bonded to assure stability. The outstanding property of these materials is their ability to act as insulators. The most important are fireclay (aluminum silicates), silica, high alumina (70-80% ALjOj), mullite (clay-sand), magnesite (chiefly MgO), dolomite (CaO-MgO), forsterite (MgO-sand), carbon, chrome ore-magnesite, zirconia, and silicon carbide. (2) Characterizing the ability to withstand extremely high temperature, e.g., tungsten and tantalum are refractory metals, clay is a refractory earth, ceramics are refractory mixtures. 

Considering the stability relationships among calcite-ankerite-magnesite, it is interesting to note that as the temperature increases the stability field... 

The stability relationships between calcite, dolomite and magnesite depend on the temperature and activity ratio of Mg " /Ca " (Fig. 5d). Lower Mg/Ca activity ratios are required to induce the dolomitization of calcite and to stabilize magnesite at the expense of dolomite (Fig. 5d) (Usdowski, 1994). Formation waters from the Norwegian North Sea reservoirs have an average log(an g -/ cz- ) - TO to 0.0 and thus fall within the stability field of dolomite. Nevertheless, both calcite and dolomite are common cements in these rocks, indicating that dolomitization is a kinetically controlled reaction. Further evidence of this is revealed from Recent sediments, such as the Fraser River delta in Canada (Simpson Hutcheon, 1995) (log (aMg2+/aca=+) -2.2 to h-1.0), where the pore waters are saturated with respect to dolomite, but it is calcite rather than dolomite that precipitates. Calcite rather than dolomite forms below the deep>-sea floor, yet the pore waters plot at shallow, near sea bottom temperatures in the stability field of dolomite and shift with an increase in depth towards the stability field of calcite (Fig. 5d). This shift is due to a diffusion-controlled, downhole decrease in Mg/Ca activity ratio caused by the incorporation of Mg in Mg-silicate that results from the alteration of volcanic material, a process which is coupled with the release of calcium (McDuff Gieskes, 1976). 

Improved refractories. The change fiom chrome-magnesite to alumina-chrome for the hearth refinctories has eliminated the susceptibility of the hearth to hydration. Previously hydration had occurred on a number of furnaces and had resulted in hearth expansion. In some cases this had seriously affected the furnace stability and its operation. 

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