Silica minerals origin

Taking into account solubility and stability boundaries of organic acids and of carbonic acid we may conclude that the organic acids are more efficient on a local scale for the dissolution of carbonate and silica cements. Where the source of the latter, i.e. the carbonic acid, is deep and further removed they may not increase the porosity by solution and removal of cement. The action of carbonic acid formed by decarboxylation on carbonate cement leads to the enrichment, in the dissolved carbonate, of the light carbon isotope inherited from the organic matter whereas the reaction of organic acids with the carbonate cement takes place under dissociation with the formation of secondary carbonate enriched with the heavy carbon isotope inherited from carbonates of mineral origin. 

Fenner, C.N. (1913) The stability relations of the silica minerals Am. J. Sci. 36, 331. Gave the original version of the silica phase diagram. 

Talc of metasedimentary origin is formed by hydrothermal alteration of a dolomitic host rock by a silica-containing fluid. This type of deposit is typical of Montana and AustraUa. It is usually quite pure with talc content of 90 to 98% and often very white as well. Dolomite [17069-72-6], CaMg(C02)2, is the most common accessory mineral. The fourth type is of metamorphic origin, where a siUcaceous dolostone is first converted to tremolite [14567-73-8] or actinohte [13768-00-8] and then partially converted to talc. The Balmat, New York, and Death Valley, California, deposits are of this type. Tremolite, dolomite, and serpentine are common accessory minerals. This type of talc deposit has a variable talc content (30—80%), but is usually white and often commercially exploited because of the properties of its accessory minerals rather than the talc. 

The weathering reactions given above show the key effects of weathering the breakdown of the original rock minerals, the consumption of H, and the release of cations and silica into solution which can then be used to make new minerals or be lost from the soil into the ground-water and rivers. 

Fig. 24.1. Volumes of minerals (amorphous silica, calcite, and sepiolite) precipitated during a reaction model simulating at 25 °C the evaporation of Sierra Nevada spring water in equilibrium with atmospheric C02, plotted against the concentration factor. For example, a concentration factor of x 100 means that of the original 1 kg of water, 10 grams remain.

Silicon undergoes internal cycling within the crustal-ocean-atmosphere fectory dm-ing which mineral silicates are produced via precipitation from seawater. These include biotic and abiotic precipitates. As compared to the crystalline silicates that ultimately originate from the igneous rocks, the precipitates are amorphous in structme. To distinguish them from the crystalline silicates, the amorphous forms are collectively referred to as opaline silica. 

The clay minerals of aeolian origin comprise 25 to 75% of the mass of pelagic sediments. The large range in composition reflects the latitudinal nature of the dust belt as well as dilution by other locally important particle types such as clay minerals of volcanogenic origin and biogenic hard parts (calcite and opaline silica). 

T,he original report by Barrer and Makki (1) that aluminum in a high A silica zeolite, clinoptilolite, could be extracted with mineral acid to give a silica pseudomorph, has given rise to considerable research on acid-extracted mordenite (2-6). Hydrogen mordenite is useful as an adsorbent and a catalyst, and its properties for some purposes are improved by partial extraction of the aluminum. Further, the ability to vary aluminum content while maintaining crystallinity offers the opportunity to leam more about the nature of the active sites in mordenite. 

Microwave heating has proven to be of benefit particularly for reactions under dry media (e.g., solvent-free conditions) in open vessel systems (i.e., in the absence of a solvent, on solid support with or without catalysts) [4]. Reactions under dry conditions were originally developed in the late 1980 s [51], but solventless systems under microwave conditions offer several additional advantages. The absence of solvent reduces the risk of explosions when the reaction takes place in a closed vessel. Moreover, aprotic dipolar solvents with high boiling points are expensive and difficult to remove from the reaction mixtures. During microwave induction of reactions under dry conditions, the reactants adsorbed on the surface of alumina, silica gel, clay, and other mineral supports absorb microwaves whereas the support does not, and transmission of microwaves is not restricted. Moreover, microwaves can interact directly with reagents and, therefore, can more efficiently drive chemical reactions. The possible accelerations of such reactions are expected... 

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