SEDIMENTARY ROCKS/Ironstones 97 Variations in the chemistry of primary fluid inclusions from ancient halite deposits are significant They also imply that seawater chemistry has changed significantly Variations are in phase with inferred seafloor spreading rates, global changes in sea-level, and the primary mineralogies of ancient marine carbonates and evaporites Of particular significance is the fact that inclusions in halites of the same age from different geological basins exhibit similar compositions This suggests that the association with dolomitization (proposed by Holland et al.) is incorrect: more interbasin variation in the amount of dolomitization would be expected, resulting in a greater variation in the chemistry of fluid inclusions than that observed It is surprising, however, that the question of whether or not variations in sodium : potassium ratios match model predictions was not addressed More recent, unpublished, work suggests that Cretaceous and Permian seawaters were enriched in potassium and relatively depleted in sodium, as would be expected from the Hardie hypothesis See Also Minerals: Sulphates Sedimentary Environments: Lake Processes and Deposits Sedimentary Rocks: Mineralogy and Classification; Dolomites Tectonics: Hydrother- mal Activity; Hydrothermal Vents At Mid-Ocean Ridges; Rift Valleys Further Reading Hanor JS (1996) Variations in chloride as a driving force in siliciclastic diagenesis In: Crossey LJ, Loucks R, and Totten MW (eds.) Siliciclastic Diagenesis and Fluid Flow: Concepts and Applications, pp 12 Special Publication 55 Tulsa: Society for Sedimentary Geology Hardie LA (1990) The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: a hypoth esis American Journal of Science 290: 43 106 Hardie LA and Spencer RJ (1990) Control of seawater composition by mixing of river waters and mid ocean ridge hydrothermal brines In: Spencer RJ and Chou I M (eds.) Fluid Mineral Interactions: A tribute to H P Eugster, pp 409 419 Special Publication San Antonio: Geochemical Society Hite RJ (1983) The sulphate problem in marine evaporites In: Schreiber BC (ed.) Proceedings of the 6th Inter national Salt Symposium, Toronto, pp 217 230 Alexandria, VA: Salt Institute Kendall AC (1989) Brine mixing in the Middle Devonian of western Canada and its possible significance to regional dolomitization Sedimentary Geology 64: 271 285 Lowenstein TK and Spencer RJ (1990) Syndepositional origin of potash evaporites: petrographic and fluid inclu sion evidence American Journal of Science 290: 42 Ironstones W E G Taylor, University of Lancaster, Lancaster, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Ironstones have been critical to industry and industrial revolutions They have been essential raw materials since the dawn of the Iron Age (about 700 bc) Without iron-rich deposits many of the manmade structures and utensils that we take for granted today – tall urban buildings, power pylons, bridges, ships, cutlery, hammers, saws, and the seemingly indispensable motor car – could not exist The point at which an ironstone deposit is considered to be an ore has changed considerably over the years and depends upon the particular economic, technological, social, and political circumstances at the time Nowadays deposits need to have an iron content in excess of 60% by weight to be worked, whilst in the mid-twentieth century ironstones with 28% iron by weight were regularly extracted as ores The quality of the potential ore, and in particular the proportion of phosphatic material, is an important factor that has to be considered in the mining of iron Initially, the availability of water power and the proximity of coal were the factors controlling production The middle of the nineteenth century saw a change from coal-fired furnaces producing cast iron to the Bessemer process, which produced steel Later in the same century, the open-hearth process and various refinements were developed Each of these new production processes demanded ores of a particular quality Records of global production are scarce before the latter half of the twentieth century, and certainly in Europe much of the exploitation predates that century In Great Britain the maximum annual output was in the order of 18 Mt (Milliontonnes) and occurred during two main periods – 1870–1890 and 1940–1945 In the former period the main type of ore extracted was from the blackband and claystone ironstones (see below) of the Carboniferous rocks of various coalfields, whilst in the latter period the