Formation Mechanism for Kaolinite
Geologically, clay minerals can be classified based on the conditions under which they form. Clay minerals can form at or near the surface of the Earth by the action of liquid water that originates either on the surface or ground water that is percolating toward the surface . Clay minerals can also form under pressure at greater depths due to the action of heated (~100-450°C) liquid-water or liquid-vapor mixtures . For both formation condition, three different mechanisms have been proposed for the conversion of aluminosilicate minerals to clays: (1) the direct reaction with water, (2) dissolution and removal of carbonate minerals, leaving insoluble clay impurities behind, or (3) the action of water on compacted shale sediments . Only the first of these mechanisms will be discussed as it pertains to formation of clays at or near the surface of the Earth, since this combination has produced the largest volumes of industrially relevant clays. In addition, only the reaction of the most common group of minerals, the feldspars, will be considered, but it is recognized that many other minerals convert to clays. To understand the source of impurities in clays, which will be discussed in the next section, the mineralogy of the rocks that serve as the aluminosilicate source are discussed in this section.
Feldspars are common aluminosilicate minerals that are present in many different igneous rocks including granites and rhyolites . When exposed, these rocks are susceptible to physical and chemical attack. Water, along with the sun, plant roots, and other forces physically attack rock formations causing crevice formation and fracture . Water also attacks rocks chemically. Over time, anhydrous aluminosilicate compounds such as those present in igneous minerals react with water to form hydrated species . The classic chemical reaction for clay formation involves the decomposition of potash feldspar due to the action of water-containing dissolved CO2 to form kaolinite (insoluble) and soluble ionic species (Reaction 1) .
In nature, the formation of clays is more complex. One complexity is due to the variable composition of feldspar and the other is due to minerals that can react to form clays . Even when only feldspars are considered, the composition can vary significantly among the end-members of the system, which are orthoclase (K2OAl2O36SiO2), albite (Na2OAl2O36SiO2), and anorthite (CaOAl2O32SiO2) . The different feldspars along with many other aluminosilicate minerals can undergo conversion to kaolinite. Another complexity is due to the fact that feldspars and other aluminosilicates are present in nearly all igneous rocks . Most often, the formation of clay is considered in the context of the decomposition of granite, a rock that contains feldspar, quartz, and mica . Quartz and mica,
which form due to incomplete decomposition of feldspar, are much more resistant to hydration than feldspar and are often left unaltered by the formation of clays from granite. As a result, quartz and mica are common impurities in primary clays.