Geology of Clays
The chapters which follow deal with the occurrence of clays in nature. This concerns the geology of clays: the geological processes which lead to the creation of clays, the transformation of clays and the destruction of clays in different geological envir
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VELDE
The chapters which follow deal with the occurrence of clays in nature. This concerns the geology of clays: the geological processes which lead to the creation of clays, the transformation of clays and the destruction of clays in different geological environments. Clays, as is the case for most objects on the Earth, are ephemeral. They have a life span which is governed by their geologic history. Clays occur under a limited range of conditions in geological space [time and temperature (essentially depth)]. They are found mainly at the surface of the Earth: their origin is for the most part initiated in the weathering (rock-atmosphere interface) environment. Some clays form at the watersediment interface (deep sea or lake bottom). A smaller number of clays form as a result of the interaction of aqueous solutions and rocks, either at some depth in the sedimentary pile or in the late stages of magmatic cooling (hydrothermal alteration). Although, this last occurrence is not of great extent it is very important to geologists as they have been called upon to aid human activity. Hydrothermal alteration often leads to the accumulation of useful heavy metals, such as gold, tungsten, and uranium. Therefore, when geologists encounter rocks which have altered to clay, and they determine that this occurred at depth (i.e. not weathering or superficial phenomena), they often take a sample in order to assess the heavy metal content. In addition to the accumulations of interesting metals, the extensive alteration of rocks due to hydrothermal alteration can produce pure clay deposits which are also of economic interest, e.g. clays for ceramics, and silica for industrial uses. Thus, as is often the case, the rare occurrence is of greater interest to humans than the more common, overall changes of geologic materials. However, in problems of environmental importance, the general, common occurrence is of greatest importance and the rare case is more of an anecdote. Environment is concerned with the everyday, while industry is concerned with the exceptions.
1.1 Why Clays Form Most clays are the result of the interaction of aqueous solutions with rocks. The dissolution and recrystallization which occurs at this encounter is the process by which clay minerals are formed and transformed. Clays are not B. Velde (ed.), Origin and Mineralogy of Clays © Springer-Verlag Berlin Heidelberg 1995
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B. Velde
stable in anhydrous environments. The proportion of water, compared to that of the solids (rock), which interacts determines the rate and type of chemical reaction and ultimately the type of clay mineral formed. When large amounts of water are present, the solids in the rock tend to be very unstable and they dissolve for the most part. Dissolution is the first step of most water-rock interactions. The greater the renewal of the water input (rain or fluid circulation) the more dissolution will occur. As the ratio of water to rock approaches one, the reactions are more and more dominated by incongruent dissolution, in which certain elem
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