Three processes that operate in the formation of bauxite and diaspore deposits are (1) desilication, in which silica is removed from clays, from feldspars, and from other aluminous minerals; (2) migration, in which clay minerals and hydrous aluminum oxides are transported within the deposit from the place where they were formed to their present position; (3) resilication, in which silica is added to gibbsite to form a hydrous aluminum silicate or clay.
The desilication of feldspars and parent rocks is usually a two-stage process in which clay minerals such as kaolinite, halloysite, and nontronite are formed first and then silica is removed from them to form gibbsite, boehmite, and diaspore. In the desilication of sedimentary clays, the original structure and texture of the clay are preserved. The formation of bauxite directly from feldspars and nepheline, which has been reported at some localities, does not appear to be the dominant process at the deposits studied during this investigation. Replacement of the clay minerals by gibbsite, boehmite, and diaspore is gradual and involves decrease in silica and increase in alumina, titania, and porosity. Transitional zones of clay-retaining structures of the parent rocks indicate that clay was an intermediate stage in the formation of many bauxite deposits. Desilication takes place in a warm moist climate through the agency of carbonic acid, alkaline carbonates, and magnesium bicarbonate solutions, either as surface weathering or by ground water diffusing through masses of clay of varying permeability and porosity.
Clay minerals and hydrous aluminum oxides migrate along cracks and open spaces in the days and bauxite. Pipes and nodules of gibbsite lacking the structure of the parent rock are formed by migration, concentration, and crystallization of secondary gibbsite.
The occurrence of cellular kaolin with a structure interpreted as inherited from gibbsite is considered evidence of resilication of gibbsite to kaolin in the bauxite district of Georgia. Dense opaque white to gray kaolinite and some of the “chimney rock” of Georgia may have formed by resilication, which takes place under conditions of poor drainage in low-lying areas.
The diaspore of Missouri, Pennsylvania, and Washington and the boehmite of Missouri, Kentucky, Washington, and California were formed from flint clays composed of finely divided kaolinite having low plasticity and breaking with a conchoidal fracture. The gibbsite of the bauxite deposits of Georgia and Alabama was formed from a kaolinitic clay of the hard or semihard type. The chief difference between the flint clays and the hard types of Georgia and Alabama is that the very fine particles of flint clay lack the degrees of symmetry in the stacking of the molecular sheets of kaolinite. The low plasticity of flint clays permitted the retention of openings and cracks without collapse, and this aided their alteration. Possible controlling conditions for the formation of gibbsite, boehmite, and diaspore include temperature, nature of the chemical systems, time, pressure, and drainage.