Mica, decomposition

Table 1. Balance of Mica Decomposition in a Weakly Podzolised Loessial Soil on Wurrn Loess (results in kg under 1 m of soil surface, after Meyer et al [1962])...

Vermiculites are formed by the decomposition of mica. They contain layers of water and magnesium ions in place the potassium ions. When heated to 800°C-1100°C, vermiculite expands because of the conversion of the water to gas. The expanded vermiculite has a low thermal conductivity and density, and is used as a thermal and sound barrier as well as an aggregate in lightweight concrete. It is also used as a moisture-retaining soil conditioner in planting. 

Thermal decomposition of iron pentacarbonyl. Very finely divided red iron oxide is obtained by atomizing iron pentacarbonyl, Fe(CO)5, and burning it in excess of air. The size of the particles depends on the temperature (580-800 °C) and the residence time in the reactor. The smallest particles are transparent and consist of 2-line ferri-hydrite, whereas the larger, semi-transparent particles consist of hematite (see Chap. 19). The only byproduct of the reaction is carbon dioxide, hence, the process has no undesirable environmental side effects. Magnetite can be produced by the same process if it is carried out at 100-400 °C. Thermal decomposition of iron pentacarbonyl is also used to coat aluminium powder (in a fluidized bed) and also mica platelets with iron oxides to produce interference or nacreous pigments. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

The mineral composition specified in Table 4 has been determined by so-called rational analysis, in which the clay is decomposed by sulphuric acid. The non--decomposed residue is considered to be quartz and feldspar (orthoclase). The feldspar content is calculated from the content of alkalis determined in the residue after decomposition with HF. The result may be affected considerably by the presence of additional minerals, since the alkalis may, for example, stem from mica. Although perfected methods of separating the individual components have been worked out, the results should always be regarded as approximate. At present, in more precise determination of mineral composition, a combination of microscopic. X-ray and thermal analyses is preferred. 

The reactions of several other minerals which thermally decompose to form mullite have been studied by Si and Al NMR. These include the mica mineral muscovite, which also contained sufficient iron to permit a complementary Fe Mossbauer study (MacKenzie et al. 1987), the hydroxyfluoride mineral topaz (Day et al. 1995) and the semi-amorphous aluminosilicate minerals allophane (MacKenzie et al. 1991) and imogolite (MacKenzie et al. 1989). The same combination of NMR nuclei has been used to study the thermal decomposition of other aluminosilicates including an illite-rich clay (Roch et al. 1998), montmorillonite (Brown et al. 1987), and a related mineral, Fuller s Earth (Drachman et al. 1997). NMR has also been used to study the effect of water vapour on the thermal decomposition of montmorillonite clay compacts (Temuujin et al. 2000a). 

Fig. 1.3 Setup for first chemical experiments with element 104 - now Rf Dubna, the mid-1960s [10]. The broken frames outline the placement of resistive heaters, paraffin and cadmium shielded the detectors from neutrons to prevent induced fission of uranium impurities in mica. Thermal decomposition of NaNbClg was the source of NbC E vapor. A Faraday cup was placed inside the target chamber (not shown).

Additives, such as mica, glass fibers, titanium dioxide, and very finely divided metals, may catalyze thermal decomposition rates during processing and should be used with caution. 

The physical properties of some fillers play a role in their function as stabilizers. A1(OH)3 undergoes endothermic decomposition which lowers temperature of material. Loss of water from MgiOH), may increase stability in some cases. In others, it may cause degradation. This is discussed below. The platelet structure of some fillers (e.g., talc or mica) contributes to an increased thermal stability because the degradation rate is increased as oxygen concentration increases. The structure formed by the platelets reduces the diffusion rate of oxygen. 

Hewitt DA, Wones DR (1984) Experimental phase relations of the micas. Rev Mineral 13 357-467 Hogg CS, Meads RE (1975) A Mossbauer study of thermal decomposition of biotites. Mineral Mag 40 79-88... 

In particular, planar defects have been investigated successfully using HRTEM and such defect structures are common in phyllosilicates, including micas. Stacking faults are also regarded as planar defects but they are closely related to polytypism, which has been described above. Here we will discuss other types of planar defects which are important for micas. They include defects related to the initial stages of the transformation of micas to other minerals, e.g., mica to chlorite, mica to vermiculite, mica to kaolinite and the decomposition of mica at high temperature. 

Isotopic. Iron-55 is an electron-capture isotope with a half-life of 2.94 years. Iron-59 is a mixed /3-7 emitter the main radiations of which areO.27 M.e.v. 8 (46%), 0.46M.e.v. /8 (54%), 1.10 M.e.v. 7 (57%), and 1.29 M.e.v. 7 (43%). The half-life of Fe is 45.1 days. The radioactive samples are most readily assayed with a standard thin-mica-window Geiger counter which will detect mainly the Fe /3-particles. Decomposition of ferrocene and ferricinium salts in a boiling mixture of four parts by volume of concentrated nitric acid and one part of 75% perchloric acid and the subsequent electrodeposition of iron on copper disks gives samples which exhibit excellent counting reproducibility. 

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