SUBJECTS dolomite

The decomposition of MgC03 (magnesite) is an interface process [734] between 813—873 K and E = 150 kJ mole-1. In the presence of C02, E was increased to 234 kJ mole-1 but was reduced slightly on the addition of ZnO or NiO. Admixture with CaO reduced the value of E to 54 kJ mole-1. This is a surprising result since the value of E for decomposition [734,753] of the mixed carbonate (Ca, Mg)C03, dolomite, is 220 kJ mole-1, larger than the value for each constituent. The influence of PCOz and of alkali metals on MgC03 decomposition has been the subject of a DTA study [404]. 

The most widely employed thermal reduction process for preparing Mg metal uses PeSi as reducing agent. Mixtures of the substrate, usually calcined dolomite (i.e., MgO, CaO) and PeSi are fabricated into briquettes with a hydrocarbon binder and loaded into Ni-Cr steel (15/28) retorts. After evacuation the retort is subjected to a temperature gradient Mg distills from the hot mixture (at 1150°C) and high-purity Mg crystals collect at the water-cooled end of the retort ... 

Bodies of Water and the Chemical Sediments ,— The chemistry of the deposition of salts from sea-water has already been made the subject of special research, and van t Hoff s results in this field are already familiar. The deposition of calcium carbonate awaits a similar thorough study. Allied questions are the formation of dolomite, the deposition of various salts from inclosed bodies of water, the deposition of phosphate rocks, the precipitation of colloidal suspensions of clay and other substances, and the origin of the great deposits of sedimentary iron ore. 

Naturally occurring talc is mined and pulverized before being subjected to flotation processes to remove various impurities such as asbestos (tremolite) carbon dolomite iron oxide and various other magnesium and carbonate minerals. Following this process, the talc is finely powdered, treated with dilute hydrochloric acid, washed with water, and then dried. The processing variables of agglomerated talc strongly influence its physical characteristics. " ... 

Two Colorado oil shale samples one from the Parachute Creek Member and the other from the C-a tract, were retorted, de-charred and then subjected to temperatures between 800 K and 1100 K in order to study the mineral reactions which take place. Comparisions between these two samples include the reversible nature of ankeritic dolomite and free calcite as well as the temperatures at which significant silication takes place. Results for the C-a tract samples indicated silication appears to take place in stages and that ankeritic dolomite decomposition can be prevented by relatively low CO2 concentrations. Ankeritic dolomite and calcite decomposition rates were similar for the two samples and there was strong evidence that calcite recarbonation takes place via non-activated chemisorption of C(>2 ... 

Fracturing of Model Minerals. Further insight into the processes responsible for liquid SO2 fracturing of these shales is provided by the behavior of several minerals when subjected to liquid SO2. Authenic samples of calcite (crystals), dolomite (hard lumps), gypsum (hard lumps), and illite (hard lumps) were treated with liquid SO2 at 25°C for 2-5 hours after removal of the SO2, the treated minerals were heated for 2 hours at 100°C infrared spectra and sulfur analyses were then obtained. Calcite crystals developed numerous fine cracks and a fine powder flaked off. Although the powder contained no sulfur (elemental analysis) or S-0 moieties (IR spectra), the cracked crystals were shown to contain 0.18% sulfur in the form of sulfur-oxygen structures. Although these data are consisted with a 0.7% conversion via... 

Reaction rates of high calcium limes are relatively high at all pH levels up to 12.4 and increase with finer particle size distributions, subject to the above comments regarding blinding by insoluble reaction products. It is for this reason that, when preparing milks of lime from quicklime, it is usually beneficial to slake under conditions which give a finely divided milk of lime (see chapter 22). The solution rates of dolomitic hydrated limes up to pH 9 also depend on particle size, but are appreciably lower than those of high calcium limes. 

In the upper part of the succession in drill hole HM-i the carbonate carbon isotope composition fluctuates between well-defined limits of -1-4.5 and -7.4%o (Table 7.1, Fig. 7.2). In this case, recrystallization of carbonates and neoformation of calcite or dolomite have taken place under participation of CO which in certain instances contained a large portion of isotopically light carbon ( C) whereas in other instances there was a predominance of heavy carbon ( C). The results obtained, together with other geochemical data, show that in the zones in which the sedimentary rocks have been affected by infiltration waters the migrating hydrocarbons have been subjected to oxidation and other changes, to be discussed in detail below. 

Fine-grained carbonates containing dolomite or siderite require a different strategy. If isotopic value of the bulk carbonate material is desired, the prepared sample can simply be reacted at a high temperature (Rosenbaum Sheppard, 1986) since the reaction rate of dolomite and siderite in phosphoric acid is very slow at 25 or even 70 °C. If isotopic values of individual carbonate minerals are desired, then this can at least be accomplished partially. The prepared material can first be reacted at 25 °C for just 1 hour, which should leave intact resistant fractions such as dolomite and siderite. The residue is then washed and dried and reacted at the requisite high temperature. In my laboratory, when this method is employed, we spot-check some samples by subjecting them to XRD-analysis both before and after the 25 °C reaction. 

Calclte (CaCO ) and dolomite Ca Mg(C03)2 are the common carbonate minerals of sedimentary (limestone, sandstone) and meta-morphic (marble) rocks used as building stones. Not only are these minerals highly susceptible to attack by acid precipitation, but the rocks which are made of these minerals also contain variable quantities of clay minerals. The chemical decomposition of the carbonate minerals as well as the contribution of clay minerals to the disintegration of these rocks is the subject of discussion in this and in the next section. 

Relatively pure carbonate rocks when subjected to regional metamorphism simply recrystallize to form either calcite or dolomite marble with a granoblastic texture. Any silica present in a limestone tends to reform as quartz. The presence of micas in these rocks tends to give them a schistose appearance, schistose marbles or calc-schists being developed. Where mica is abundant, it forms lenses or continuous layers, giving the rock a foliated structure. 

Tar-impregnated Refractory. A fired refractory, particularly one subject to hydration, such as dolomite or magnesite, whose pores have been filled with tar by autoclaving at c. 200°C, evacuating and pouring in tar. Tar impregnated refractories are used to line basic oxygen furnaces. 

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