Silica minerals transformation

As an example of these ideas, an activity-ratio diagram for control of Al(III) solubility by secondary minerals in an acidic soil solution will be constructed. The Jackson-Sherman weathering scenario14 indicates that when soil profiles are leached free of silica with fresh water, 2 1 layer-type clay minerals are replaced by 1 1 layer-type clay minerals, and ultimately these are replaced by metal oxyhydroxides. This sequence of clay mineral transformations can be represented by the successive dissolution reactions of smectite, kaolinite, and gibbsite ... 

The most widespread of the secondary minerals formed during the development of a laterite weathering profile are iron and aluminium ses-quioxides (Table 3.1). These may form either directly from the alteration of primary minerals, or else via a series of pathways involving the formation of intermediary sheet silicate minerals and clays (e.g. chlorite, illite, smectite, vermiculite and halloysite), which are then themselves broken down, stripped of their mobile ions and silica, and eventually converted to alumina and ferric oxyhydroxide residua (Figure 3.9). It is not possible to describe these mineral transformations in detail, but the key issue is that under tropical-type weathering conditions these transformation pathways lead to... 

Octahedral stishovite is 46 % denser than coesite. It has a specific gravity of 4.35 for the synthestic material. In stishovite each oxygen is co-ordinated to three Si, as opposed to two " Si in lower-pressure silica minerals. The Si —> Si transitions increase the Si-0 separation as well as the packing efficiency. The Si — Si transformations occur over a limited range of pressure. 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

Carbon tetrachloride slowly reacts with hydrogen sulfide in aqueous solution yielding carbon dioxide via the intermediate carbon disulfide. However, in the presence of two micaceous minerals (biotite and vermiculite) and amorphous silica, the rate transformation increases. At 25 °C and a hydrogen sulfide concentration of 0.001 M, the half-lives of carbon tetrachloride were calculated to be 2,600, 160, and 50 d for the silica, vermiculite, and biotite studies, respectively. In all three studies, the major transformation pathway is the formation of carbon disulfide. This compound is... 

For minerals featuring modifying transformations with temperature increase, the decline in hardness is not continuous in character—a temporary increase in hardness takes place in the vicinity of the transformation temperature. This phenomenon is presented in Fig. 6.4.8 (curve 4) for a quartz sample. It is also true for silica glass processed by quartz melting. The curves for silica glass (Fig. 6.4.8, curves 1, 2, 3) show two breakdowns... 

Rapid increase in temperature is desirable at temperatures below those at which substantial liquid formation occurs (C9,B27,S21,C11,W9,G26). Most of the belite, and almost all of the other product phases, subsequently either melt or react in the presence of the melt, and there is no merit in promoting crystal growth or removal of imperfections, which would impede these processes. Slow heating may also allow the decomposition products of the clay minerals to transform into less reactive phases. It can also lead to the formation of microstructures unfavourable to the later reactions Chromy (C9) found that it allowed the belite shells around the silica particles to thicken, producing composites slow to react with lime. In contrast, rapid heating increases movement of the liquid phase, when this forms, and thus improves the mixing of the calcareous and siliceous constituents (Cl 1). 

The rate of diagenetic transformations of iron cherts is determined not only by T and P, but also by the particulars of deposition, form of migration, dispersion of the particles, and pH of the environment. The time it takes to transform amorphous sediments into crystalline minerals is very great. For iron hydroxides it is several million years, and for silica it may be still more. 

Transformation of Biogenic Silica to Authigenic Clay Minerals... 

Opal-CT is a poorly ordered silica polymorph composed of interstratified cristobalite and tridymite layers (Jones and Segnit, 1971, 1972) it is the mineral which is often misidentified as cristobalite in studies of the diage-netic transformations of silica. 

When the clay mineral kaolinite is heated to about 980°C, it transforms to an intimate mixture of amorphous silica and an aluminium-rich spinel phase, the latter having potentially useful catalytic properties if the silica can be removed. Selective leaching... 

Bmno E, Pentinghaus H (1974) Substitution of cations in natural and synthetic feldspars. In MacKenzie WS, Zussmarm, J (eds) The Feldspars. Manchester Uitiv Press, Manchester, p 574-609 Buerger MJ (1951) Crystallographic aspects of phase transformations. In Smoluchowski R (ed) Phase Transformations in Solids. Wiley, New York, p 183-211 Buerger MJ (1954) The stuffed derivatives of the silica stiuctures. Am Mineral 39 600-614 Bulenda M, Schwabl F, Tauber UC (1996) Defect-induced condensation and the central peak at elastic phase transitions. Phys Rev B 54 6210-6221... 

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