Kaolin: Soil, rock and ore

The kaolinite-group minerals kaolinite, dickite, nacrite, halloysite including its metaform and their associates allophane and imogolite are phyllosilicates characterized by a rather simple chemical composition of Si, Al, O, and H. These elements contribute for the most part to the built-up of the continental earth crust, which down to a depth of approx. 1.5 km, consists of 75% of sedimentary rocks. In a sense, kaolinite-group minerals accommodating these elements in their structure chemically reflect the uppermost part of the crust. It is not a surprise that kaolin is very widespread particularly in those sedimentary rocks which came into existence under near-ambient conditions and, as a further consequence, is a mirror image of those processes taking place in the topmost parts of the crust. In a tripartite subdivision (primary: magmatic/structure bound, secondary: sedimentary, tertiary: metamorphic), the following environments bearing kaolin exist: Primary environments of kaolinization: (1) Vein-type deposits, (2) (sub) volcanic and pyroclastic deposits, (3) skarn to epithermal deposits, (4) granitic rocks and their affiliated rocks (pegmatites and greisen). Secondary environments of kaolinization:(1) kaolin and soil (ferralsols, plinthosols, nitisols, podzols, vertisols, andosols), (2) layered residual kaolin deposits (mixed-type residual kaolin-bauxite deposits, exposed residual kaolin, hidden residual kaolin), (3) vein-like kaolin, (4) alluvial-fluvial environments (alluvial fans, fluvial braided streams, fluvial meandering streams),(5) prograding fluvial deltas (prograding into a playa (dry delta), prograding into a marine or lacustrine basin (wet delta)),(6) lakes and ponds (permanent and ephemeral lakes), (7) coal-bearing environments (suspended load deposits in coal swamps, underclays, composite residual and hydrothermal kaolinization in coal swamps, kaolin tonsteins), (8) marine terrigenous shoreline deposits (open - tide-dominated estuary, blind - wave-dominated estuary, sealed-off lagoon). Tertiary environments of kaolinization: (1) burial diagenesis, (2) very-low grade regional dynamo metamorphism, (3) contact metamorphism. The above tripartite subdivision of kaolin has been established so as to be in accordance with other lithologies which formed through magmatic, sedimentary, and metamorphic processes and to link the present classification scheme directly with the “Chessboard classification scheme of mineral deposits” (Dill, 2010b). While in many classification schemes of mineral deposits kaolin and its minerals were only considered as an “ore” in the category non-metallic deposits and industrial minerals, in the current review the barriers between economic geology and its neighboring disciplines like sedimentology, pedology, geomorphology, petrography and palaeoclimatology have been torn down and the kaolin looked at from different angles, as soil, rock and ore. Kaolin and kaolinitic clays are taken as the type-lithology of the near-surface continental environments. Together with bentonites, bentonitic clays and a varied spectrum of argillites they form part of a group of lithologies, encompassing besides bauxite, ferrites (ferricretes) and laterite, all of which developed close to the interface between atmosphere, pedosphere, hydrosphere, and lithosphere. To accentuate the intimate relation between the various lithologies mentioned in the previous paragraph, a classification scheme has been designed. It makes use in part of pre-existing ternary plots to take also account of these interferences with the different geoscientific disciplines such as sedimentology or pedology. In addition to that, an overview of the various field and laboratory methods to identify and quantify kaolin/kaolinite-group minerals is given. The succeeding parameters, features and settings are crucial as to the kaolinization in the magmatic, sedimentary and metamorphic environments.