30 SEDIMENTARY ROCKS/Mineralogy and Classification also commonly contain varying amounts of clay matrix and organic matter Claystones are composed largely of the clay minerals, kaolin, illite, montmorillonite, and chlorite Clay mineralogy is described in detail in Clay Minerals Lime and organic matter may also be present in claystones With increasing lime content, shales grade into marls, argillaceous limestones, and limestones With increasing organic content, shales grade into sapropelite Detrital grains of silt, mica, plant debris, and shell fragments may occur in mudstones as impurities Clays and their diagenesis are described in Sedimentary Rocks: Clays and Their Diagenesis Autochthonous or Chemical Sediments It has already been noted that the classification of sediments into detrital and chemical categories is somewhat artificial Both are composed of chemical or biochemical components, and of particles of varying sizes Nonetheless, it is a convenient grouping The chemical sediments are those that principally precipitate out of solution, although thereafter they may become detrital in some instances Table shows that seven types may be recognized: carbonates, evaporites, residual deposits, kerogen, ironstone, phosphate, and silica These are now described briefly Carbonates The carbonate chemical sediments include a wide range of rocks, of which limestone and then dolomite are volumetrically the most important, whilst siderite and magnesite, although rare, are economically important The mineralogy of carbonates is described in detail in Minerals: Carbonates Limestone is composed largely of the mineral calcium carbonate (CaCO3) Limestones may be made up of many different types of grain that originate in different ways in a range of depositional environments (Figure 6) Thus grain type is one of the keys to interpreting their depositional environment Limestones form, almost without exception, from the aqueous precipitation of calcium carbonate, aided by some organic process or other, most obviously as shells secreted by invertebrates, but also as nodules, laminae, and clouds whose origins owe much to biochemical reactions Limestones are described in greater detail in Sedimentary Rocks: Limestones Dolomite is composed of the mineral dolomite, CaMg(CO3)2, named by and from the eponymous French Count Dolomieu (1750–1801) Geopedants restrict the term dolomite to the mineral, and dolostone to the rock This was not the Count’s original intent Like limestone, dolomite forms in several different ways, from penecontemporaneous cryptocrystalline mudstones in sabkhas, to coarsely crystalline varieties during late diagenesis In the latter case, dolomite is virtually a metamorphic rock (Figure 7) Dolomite is an important petroleum reservoir, and occurs as a gangue mineral with lead–zinc sulphide ores (see Mining Geology: Hydrothermal Ores) It is described more fully in Sedimentary Rocks: Dolomites Figure Photomicrograph of limestone under ordinary light This is a well sorted oolite grainstone from the Upper Jurassic Portland Limestone, Dorset, UK Reproduced with permission from Selley RC (2000) Applied Sedimentology, 2nd edn London: Academic Press