Silica time-dependent evolution

There is, in addition, a readily recognizable rehydration rate dependence on the surface siloxane content. This is most apparent in the immersion heat data for silica FS, for which the time of slow heat evolution on immersion becomes appreciably longer for sample pretreatment temperatures above 225° C. Consideration of the precision of the calorimetric measurement with regard to slow heat input does not seem to have been afforded adequate attention. Immersion heat calorimeters constructed for the essentially instantaneous wetting heat measurement often do not perform adequately when heat is evolved slowly. 

These two distinct processes lead to the formation of secondary minerals mainly phyl-losilicates such as clays, of soluble products (e.g., carbonates or silica) lixiviated by percolating waters and of colloids usually iron and aluminum sesquioxides complexed by humic acids. While physical degradation involves mechanical (e.g., abrasion, impact) or thermal (e.g., thermal shock) processes, alteration involves only chemical reactions such as hydrolysis influenced by pH conditions and/or the oxidation of primary materials depending on the Eh (redox potential) conditions. Whatever the type of underlying rock, the end product is always a clay except when silica is totally absent from the bedrock, the composition of the clay depending on the type of climate and the time over which the evolution process takes place. These conditions are summarized in Table 14.1. 

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