102 SEDIMENTARY ROCKS/Ironstones phosphates, coals, evaporites or laterites, and most have no direct relation to volcanism Blackband ironstones have many of the characteristics of bog iron ores, which are developed in situ, soon after deposition, by a reaction between organic material and underground colloidal iron-rich solutions under a thick vegetative cover Progression of the process could yield siderite by reduction Alternative evidence has been put forward suggesting that these deposits could form by direct sideritic precipitation from tropical swamp waters that are already rich in iron Blackband ironstones are always developed in close proximity to coal seams, so either process could be feasible The diagenesis of the fine-grained claystone ironstones has been studied in great detail (Figure 6) Most became enriched in iron during very early diagenesis along or near the sediment–water interface Based upon distinct chemical reactions involving the oxidation of organic matter buried within the sediment, diagenetic zones have been established Although the zones can be considered as due to burial, their development is especially dependent upon the availability of oxidizing agents and organic matter, the sedimentary environment, the nature and amount of organic material, the composition of the inorganic sediment, the hydrological regime of the sedimentary pile, and the composition of the overlying water The reactions below the oxic zone may be complicated by kinetic controls, which could explain the occasional appearance of residual ferric iron in an anoxic environment Because some siderite concretions are developed early and are associated with many nonsequences, the sedimentation rate must have been relatively low (less than 40 m Ma 1) Whilst claystone ironstones are formed during diagenesis by the growth of siderite in the pore spaces of argillaceous materials, sphaerosiderites form by the direct precipitation of siderite from pore fluids, and their size and shape probably reflect a higher growth rate They can occur in a variety of environments, including the deep sea, but are usually products of a waterlogged zone below a leached soil profile The exact genesis of ooidal ironstones remains controversial Particularly, the origin of the ooids is the subject of a long-lasting debate The original constituents of ooids and how they vary from deposit to deposit are not known with any certainty It is debatable whether the ooids grew from solutes, colloidal particles in solutions, or gels The ferrous ion in bicarbonate form survives only in an anoxic or reducing environment, so this would place a severe constraint on its presence in solution Ferruginous ooids are commonly built of alternating ferric oxide and berthierine sheaths of submicroscopic thickness Whether the initial crystalline phase was berthierine Figure Summary of reactions and zonation that may occur during the diagenesis of sediments in marine and non marine conditions (after Curtis and Coleman 1986, Spears 1989 and reproduced with kind permission of Kluwer Academic Publishers from Young TP (1993) Sedimentary iron ores In: Pattrick RAD and Polya DA (eds.) Mineralization in the British Isles, pp 446 489 London: Chapman & Hall, Figure 9.5 after Curtis and Coleman 1986 and Spears 1989)