Magnesite, 856 table

The interpretation of the C.E. by a superimposition of reactions occurring at different active surface centers is compatible with the fact that many multicomponent catalysts exhibit a C.E. but no C.E. is found when very pure substances have been subjected to different thermal pretreatments (17). This implies the possibility that many active centers are due to impurities and that their numbers may change with the pretreatment of the catalyst, e.g., by means of aggregation, volatilization, etc. As an illustration, data for the decomposition of N2O on MgO, prepared from synthetic and from natural magnesites, and data for the para-ortho hydrogen conversion on pure metals and on alloys are presented in Tables II and III. 

A classic example of metastability is surface-seawater supersaturation with respect to calcite and other carbonate minerals (Morse and Mackenzie 1990 Millero and Sohl 1992). The degree of calcite supersaturation in surface seawater varies from 2.8- to 6.5-fold between 0 and 25 °C (Morse and Mackenzie 1990). In Fig. 3.18, experimental calcite solubility (metastable state) is approaching model calcite solubility (stable state) at subzero temperatures. In Table 5.1, the difference in seawater pH, assuring saturation or allowing supersaturation with respect to calcite, is 0.38 units. Moreover, in running these calculations, it was necessary to remove magnesite and dolomite from the minerals database (Table 3.1) because the latter minerals are more stable than calcite in seawater. But calcite is clearly the form that precipitates... 

The applicability of scanning Auger spectroscopy to the analysis of carbonate mineral surface reactions was demonstrated by Mucci and Morse (1985), who carried out an investigation of Mg2+ adsorption on calcite, aragonite, magnesite, and dolomite surfaces from synthetic seawater at two saturation states. Results are summarized in Table 2.5. 

The following test materials have often been used FCC catalysts, aluminium oxide, silica gel, glass beads, silica or quartz sand, sea sand, coal and coal ash, petroleum coke, metal powders, resin particles, boric acid, and magnesite powder. Mean particle size ranges from 11 /un to 1,041 /rm, and particle density, from 384 kg/m3 to 7,970 kg/m3. According to Geldart s classification (1973), most of these materials belongs to Class A, some to Class B, and a few to Class D or C, as listed in Table II. 

Magnesium. Mg, at wt 24.312, at no 12, valence 2. Isotopes 24 (77.4%), 25 (11.5%) 26 (11.1%). Physical properties of 99.9% pure Mg are given in the following table (Ref 19, p 679) Mg is very abundant in nature, occurring in substantial amounts in many rock-forming minerals such as dolomite, magnesite, olivine, and serpentine. In addition, it is also found in sea water, subterranean brines, and salt beds. 

Barium is a member of the alkaline earth metals. The alkaline earth metals make up Group 2 (IIA) of the periodic table. The other elements in this group are beryllium, magnesium, calcium, strontium, and radium. These elements tend to be relatively active chemically and form a number of important and useful compounds. They also tend to occur abundantly in Earth s crust in a number of familiar minerals such as aragonite, calcite, chalk, limestone, marble, travertine, magnesite, and dolomite. Alkaline earth compounds are widely used as building materials. 

Magnesite refractories exhibit resistance to high-temperature deformation and to corrosion attack by basic melts. Some of their properties are listed in Table 33. 

Natural talc in usually contaminated with accessory minerals, mainly magnesite and minerals containing iron oxides. The contaminated raw materials are treated by flotation and magnetic separation. Ceramic manufacture requires low contents of Fc203 and CaO (each below 1.5%) and the highest possible MgO content. The composition of natural talc is shown in Table 8,... 

Thci ch( inical composil ion of the several types of magnesite is given in the following table ... 

Magnesite is the most common magnesium carbonate mineral in geological environments. In spite of this fact, it is rarely observed precipitating from natural waters. Further, its low-temperature solubility has been extremely difficult to measure because of its very slow rate of dissolution in the laboratory (cf. Langmuir 1965). The - log of 4.9 for magnesite given in Table 6.1 is consistent... 

TABLE 2.2 Worldwide Reserves of Magnesite (Million Tonnes)... 

In addition to those deposits listed in Table 2.2, magnesite is also found in Spain, Pakistan, and the Sudan. Deposits with reported reserves of less than one million tonnes occur in Mexico, Iran, the Philippines, Australia, Egypt, and the Republic of South Africa. Small deposits have also been found in Cuba, Norway, Sweden, Poland, Scotland, France, Italy, Kenya, and Tanzania. The dominant producers of magnesite are North Korea, China, Slovakia, Turkey, Russia, Austria, and India, which together account for 75% of world output. For a more complete listing of other minor occurrences of magnesite and brucite the reader is referred to www.mindat.org. 

Products and Uses An additive in inks, dentifrices, cosmetics, free-running table salt, antacid, and foods. Used as a drying, color-retaining, and anticaking agent. It is the synthetic form of mineral magnesite. 

These results show that significant modifications appear, but that they can be considered as perturbations caused by the crystal field. They only weakly alter the properties of ions. In calcite or magnesite, there is a transfer from carbonate ions toward the metal l[Table 10), which modifies the number of electrons for the carbonate system. To estimate the corresponding effect on the geometry, we shall use the molecular orbitals corresponding to the isolated ions The loss of 0.1 electron, which affects the two highest... 

TABLE 1. Exponents (bohr ) and contraction coefficients of the Gaussian functions adopted for the study of MgCO 3, magnesite (m) and CaCOs, calcite (c). The contraction coefficients multiply individually normalized Gaussians. y [ z ] stands for y x 10. ... 

TABLES. Hartree-Fock energies (including relaxation contribution), and relaxed c/a ratios and oxygen fractional coordinates vs. V/Vo for magnesite and calcite. The Vq/6 (least-energy volume per formula unit), Ef) = E(Vo), Bq (zero-pressure bulk modulus) and B = (dB/dp)o- 0 quantities have been fitted to a Murnaghan equation of state. 

Figure 4. Computed interatomic distances of calcite and magnesite, divided by the values at p-0 (Table 4), plotted versus pressure.

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