98 SEDIMENTARY ROCKS/Ironstones Figure Ordovician ironstones, Betws Garmon, North Wales, UK (inset showing the steeply inclined stoping method of under ground mining for the deposit and a residual pillar of magnetite rich material) ooidal ironstones of Northamptonshire and Lincolnshire were the dominant ores Although Ordovician ooidal ironstones from North Wales were extracted until early part of the twentieth century (Figure 1) The terms used to describe both the processes of ironstone formation and the ironstones themselves have been many and varied Attempts to simplify and standardize the terminology have recently met with some success, mainly through the International Geological Correlation Programme (IGCP) For example, the terms ‘Clinton’ (from the Silurian Clinton Group of New York State, USA) and ‘Minette’ (from the Jurassic Minette oolitic ironstones of northeastern France and adjacent areas) as descriptions of ironstone types have proved to be unsatisfactory and have now fallen into disuse Definition Largely because iron may invade and impregnate a wide range of rocks, defining what constitutes an ironstone has proved difficult Exhortations to merge the nomenclatures of the various iron-rich deposits, such as the banded iron formations and ironstones, have been resisted on the basis that the mineralogy, petrology, and genesis of these deposits are distinct and separate A precise definition of ‘ironstone’ was agreed only in the last decade of the twentieth century and stems from the work of the IGCP 277 (Phanerozoic Ooidal Ironstones) Ironstones are sedimentary rocks consisting of at least 15% iron by weight, which may be quoted as 19% FeO or 21% Fe2O3 or an equivalent admixture in a chemical analysis They occur almost exclusively in the Phanerozoic Era and are distinguished from the mainly Precambrian banded iron formations (see Sedimentary Rocks: Banded Iron Formations) by their lack of both regular banding and chert and by their age: banded iron formations were produced when there was a deficiency of oxygen in the Earth’s atmosphere The ferruginization (iron enrichment) may be the result of either direct deposition or subsequent chemical changes Ironstone Mineralogy The iron-ore minerals may be oxides (including goethite, haematite, and magnetite), carbonates (usually siderite), or silicates (normally berthierine or chamosite) They may be associated with other carbonate minerals, sulphides and/or phosphatic minerals Goethite – FeO(OH) – is commonly formed by oxidation during weathering Also, in many ooidal ironstones, it can result from the oxidation of berthierine; the two minerals may be found intermixed, often in alternate concentric layers, in ooids Limonite was formerly thought to be a distinct mineral with the composition 2Fe2O3 Á 3H2O but is now considered to have a variable composition (and properties) and to consist of several iron hydroxides (commonly goethite) or a mixture of iron minerals Generally, it occurs as a secondary alteration product Haematite – Fe2O3 – can be an important mineral in some ironstones, where it is usually formed as a late-stage