32 SEDIMENTARY ROCKS/Mineralogy and Classification Figure Photomicrograph of nodular anhydrite (CaSO4) from the sabkha of Abu Dhabi, United Arab Emirates Reproduced with permission from Selley RC (2000) Applied Sedimentology, 2nd edn London: Academic Press a nodular pisolitic texture They are often termed ferricrete (etymologically, although not genetically, comparable with silcretes and calcretes) Laterites are important sources of iron in West Africa and Western Australia, and of nickel in Cuba Further details are given in Soils: Modern and Soils: Palaeosols China clay, or kaolin, is the name given to rock composed almost entirely of the clay mineral kaolinite, Al2Si2O5.(OH)4 (see Sedimentary Rocks: Clays and Their Diagenesis) Kaolin and kaolinite are occidental corruptions of Kauling, a hill in China, from whence the first samples to enter Europe were shipped by a Jesuit missionary in 1700 Some kaolinite is produced by the in situ hydrothermal alteration of feldspar in granites, as for example that of southwest England Kaolinite may then be reworked from such a source, and re-deposited in lacustrine environments, as for example the Oligocene ‘Ball Clay’ deposits of Bovey Tracey, Devon Kaolin also forms, however, as a residual deposit due to the intense weathering of aluminosilicate-rich rocks These include feldspar-bearing igneous rocks, such as granites and gneisses Kaolin can also be produced from sedimentary rocks, including arkosic sandstones and shales The general chemical reaction leading to the production of kaolinite is: KalSi3 O8 ỵ H2 O ! Al2 Si2 O5 OHị4 Feldspar ỵ Water ! Kaolinite Kaolin forms as a residual deposit in the soil horizons of warm humid climates, where erosion rates are low, and there is plenty of time for leaching to take place Kaolin has many important industrial uses (see Clays, Economic Uses) Notable commercial deposits occur in China, naturally, south-west England, Saxony (Germany), Bohemia (Czech Republic), and Georgia, USA The third type of residual deposit is bauxite, hydrated aluminium hydroxide (Al2O4.nH2O) Bauxite takes its name from Le Baux, near Arles in France Bauxite is the end result of the intensive and prolonged weathering of soils that commences with laterite, and proceeds, via kaolin, to bauxite These changes reflect the progressive leaching of silica, iron, and kaolinite (Figure 9) The chemical reaction that finally leads to the formation of bauxite is: H2 O ỵ Al2 Si2 O5 OHị4 ẳ Al2 O4 :nH2 O ỵ 2SiO2 :2H2 O Water ỵ Kaolinite ẳ Aluminium Hydroxide ỵ Silicic Acid Bauxites tend to be reddish or pink in colour due to some residual iron oxide They may also possess a pisolitic texture inherited from an earlier lateritic phase (Figure 10) Bauxite is very important as the ore for aluminium Bauxite occurs as residual deposits on limestone, as for example in France and Jamaica It also occurs as a residual soil on Precambrian igneous and metamorphic rocks, as in Surinam