SEDIMENTARY ROCKS/Mineralogy and Classification 31 Figure Photomicrograph of dolomite under ordinary light This is a coarsely crystalline variety from the Zechstein (Upper Permian) of the UK North Sea Some porosity (pale blue) is visible Reproduced with permission from Selley RC (2000) Applied Sedimentology, 2nd edn London: Academic Press Siderite is composed of iron carbonate (FeCO3) It occurs commonly in shales as early cement and as concretions It occurs as crystalline cement in sandstones, and occasionally in spherulites (sphaerosiderite) in lacustrine deposits Here, it may be sufficiently abundant to become an iron ore (described below) Magnesite (MgCO3) is the name for the mineral magnesium carbonate, as well as the rock It forms both as an alteration product of dolomite, and from the action of magnesium-rich fluids on limestone Magnesite is rare, but occurs in commercially important deposits at Radenhein (Austria), Liaotung (China), and Clark County, Nevada (USA) Evaporites The evaporite chemical sedimentary rocks are rare, but extremely important commercially as the raw materials for the chemical industry As the name suggests, the evaporites consist of a suite of minerals formed from the evaporation of sea water They tend to occur in restricted sedimentary basins in cyclic sequences that begin with carbonates (limestone and/ or dolomite), overlain by sulphates (gypsum and/or anhydrite), halite (sodium chloride), and then a range of potassium salts, including carnallite and polyhalite (Figure 8) As the name suggests, it was once thought that evaporites formed exclusively from the drying out of enclosed marine basins This required improbably large volumes of sea water to provide the resultant evaporites It is now realized that many evaporites actually form in sabkhas (Arabic for salt marsh) from the replacement of pre-existing rocks, principally carbonates, by circulating brines Evaporites should thus more correctly be termed ‘replacementites’ Evaporites are described in more detail in Sedimentary Rocks: Evaporites Residual Deposits Residual deposits are a variety of rocks produced by in situ chemical alteration or weathering (see Weathering) They include three economically important rocks: laterite, china clay (kaolin), and bauxite These are now described in turn The word ‘laterite’ is derived from the Latin ‘later’, a brick, as this rock has been widely employed for this purpose, being soft when quarried, but hardening on exposure The term was first employed by a British geologist of the Raj, working in India, where laterites are exceptionally well developed Laterites result from the intense weathering in many parts of the world of rocks of diverse ages and types, but particularly iron-rich rocks such as basalts Laterites thus occur as laterally extensive residues up to 10 m in thickness above the bedrock They require thousands of years to form, a humid climate, and a well-drained terrain The resultant laterite is rich in hydrated iron and aluminium oxides, and low in humus, silica, lime, silicate clays, and most other minerals Laterites are red and argillaceous in appearance, but often possess