Weathering diagenetic alteration

The difference in the composition of these two size fractions is similar to the difference between the two types of clay vermiculite described by Barshad and Kishk. The two sets of data confirm the idea that clay vermiculites developed by mild leaching action of pre-existing sheet structures tend to inherit much of their octahedral and tetrahedral character. Clay vermiculites formed by relatively intense weathering and by diagenetic alterations and in approximate equilibrium with their soil environment will have little if any tetrahedral Al the octahedral sheet can be quite variable in composition and depends on the availability of Al, Fe, and Mg. 

Mixed-layer clays form by the alteration of pre-existing micas and illites, by hydrothermal action, by the alteration of volcanic glass, and by diagenetic alteration of montmorillonite. During continental weathering K is leached from micas and illites and mixed-layer clays are formed. When these clays are carried to the sea, they may adsorb K and revert partially or entirely to illite. If they remain in a continental environment where K may not be available, the expanded layers can persist until K... 

In light of the small solubilities of many minerals, the extent of reaction predicted by this type of calculation may be smaller than expected. Considerable amounts of diagenetic cements are commonly observed, for example, in sedimentary rocks, and crystalline rocks can be highly altered by weathering or hydrothermal fluids. A titration model may predict that the proper cements or alteration products form, but explaining the quantities of these minerals observed in nature will probably require that the rock react repeatedly as its pore fluid is replaced. Local equilibrium models of this nature are described later in this section. 

The geological environments which form clay minerals can be basically divided into five types weathering, sedimentation, burial, diagenetic and hydrothermal alteration. The weathering environment frequently presents a chemical system where T,P are constant and many chemical elements are mobile, usually they enter solution from the rocks present at the earth s surface through the process of hydrolysis. The major problems are (a) Determination of rates of reaction among the minerals present,... 

Vermiculite is a widespread hydrated clay mineral of lesser abundance than smectite. Understanding of its diagenetic behavior is complicated by the fact that most of the laboratory measurements on vermiculite have been made on the hydrothermal alteration products of coarse-grained biotites, whereas most soil vermiculites that would be fed into a sedimentary pile like that of the Gulf of Mexico coast are weathered dioctahedral Ulites containing a lot of interlayer Al- and Fe-hydroxides. 

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