SEDIMENTARY ROCKS/Limestones 107 Curtis CD and Coleman ML (1986) Controls on the precipitation of early diagenetic calcite, dolomite and siderite concretions in complex depositional sequences In: Gautier DL (ed.) Roles of Organic Matter in Sedi ment Diagenesis, pp 23 33 Special Publication 38 Denver: Society of Economic Palaeontologists and Min eralogists Curtis CD and Spears DA (1968) The formation of sedimentary iron minerals Economic Geology 63: 257 270 Kearsley AT (1989) Iron rich ooids, their mineralogy and microfabric: clues to their origin and evolution In: Young TP and Taylor WEG (eds.) Phanerozoic Ironstones, pp 141 164 Special Publication 46 London: Geological Society of London Kimberley MM (1994) Debate about ironstone: has solute supply been surficial weathering, hydrothermal vection, or exhalation of deep fluids? Terra Nova 8: 116 132 Odin GS (ed.) (1988) Green Marine Clays, Oolitic Iron stone Facies, Verdine Facies, Glaucony Facies and Celadonite Bearing Facies A Comparative Study Devel opments in Sedimentology 45 Amsterdam: Elsevier Petranek J and Van Houten F (1997) Phanerozoic Ooidal Ironstones Special Papers Prague: Czech Geological Survey Spears DA (1989) Aspects of iron incorporation into sedi ments with special reference to the Yorkshire Ironstones In: Young TP and Taylor WEG (eds.) Phanerozoic Ironstones, pp 19 30 Special Publication 46 London: Geological Society of London Taylor JH (1949) The Mesozoic Ironstones of Britain: Petrology of the Northampton Sand Ironstone Memoir of the Geological Survey of Great Britain London: Geo logical Survey of Great Britain Van Houten FB and Arthur MA (1989) Temporal patterns among Phanerozoic oolitic ironstones and oceanic anoxia In: Young TP and Taylor WEG (eds.) Phanero zoic Ironstones, pp 33 49 Special Publication 46 London: Geological Society of London Young TP (1993) Sedimentary iron ores In: Pattrick RAD and Polya DA (eds.) Mineralization in the British Isles, pp 446 489 London: Chapman & Hall Young TP and Taylor WEG (eds.) (1989) Phanerozoic Ironstones Special Publication 46 London: Geological Society of London Limestones R C Selley, Imperial College London, London, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Limestones are one of the most important of all the sedimentary rocks introduced in (see Sedimentary Rocks: Mineralogy and Classification) Limestones are composed largely of calcium carbonate (CaCO3) in the mineral form calcite, but there are several other important carbonate minerals with which limestones are associated This article opens by discussing important differences between limestones and sandstones, and continues by outlining the mineralogy, classification, and rock names of limestones This is followed by a brief account of limestone depositional environments, and, logically, by their postdepositional diagenesis The article concludes with a description of the economic importance of limestones, which is considerable, and a selected reading list Differences between Limestones and Sandstones Limestones and sandstones are the two most important groups of sedimentary rocks However, limestones pose a completely different set of problems to those of sandstones, the solutions of which require the application of different concepts and techniques First, limestones, unlike sandstones, are intrabasinal in origin That is to say they form in the environment in which they are deposited The source material of sandstones, by contrast, has been weathered, eroded, transported, and may finally be deposited hundreds of kilometres from its point of origin Sandstones (or siliciclastic rocks) therefore often contain many different minerals Limestones, by contrast, have a much simpler mineralogy, generally consisting of only calcite and two or three others (which will be mentioned shortly) Siliciclastic sand grains may hold clues to their source, but tell little of their depositional environment Limestone grains, by contrast, although largely monomineralic, occur in a wide range of sizes and shapes, reflecting their multiple origins These grains form in specific environments from which they are seldom transported Limestone grains thus give important clues about their environment of deposition When studying sandstones, vertical profiles of grain size and analysis of sedimentary structures are the keys to environmental diagnosis With limestones, however, it is the analysis of grain type and texture that aids environmental diagnosis The second large difference between sandstones and limestones lies in their chemistry Sandstones are