Tom Kearsley – Mineralogist
Iron is arguably the most important metallic element in the history of human technology. In the most comprehensive modern reference volume on properties, processing and use of metals – the Metals Handbook edited by Davis, 1998 – there are more pages devoted to ‘ferrous’ metals (‘irons’, steels and high performance alloys) than to all of the other metals combined.
Together with Magnesium (Mg) and Aluminium (Al), Iron (Fe) is an abundant element throughout the Solar System (Lodders, 2010), including the Earth. It was inherited from dust created by ancient giant stars, then brought together over four and a half billion years ago during the formation of the planet from the collision of asteroids and meteorites in the early Solar System. Much of the Earth’s Fe, along with Nickel (Ni) and Sulfur (S), is now in the core where it is responsible for the magnetic field of the planet. ‘Iron’ is also occasionally found on Earth’s surface as a ‘native’ metal, this may come from meteorite falls (which will not be pure Iron element, but will also contain a little Ni), and even a little can be found in some volcanic lavas. This raw material has been used by people for at least 5000 years, but it is so rare that ‘iron’ was not the most widely used metal until much later. In nature, Mg and Al readily form common minerals with Silicon (Si) and Oxygen (O), but they are not found as metals without human intervention, and they have only become widely manufactured and used in the last century.
Although now a little dated, ‘Metals in the Service of Man’ by A. Street and W. Alexander (10th edition, 1994) provides a concise and readable introduction to the sources of metals, their processing, properties and uses. An excellent and detailed explanation of how metals (including ‘irons’) came to be produced, from the earliest methods up to modern large-scale industries, can also be found in ‘A History of Metallurgy’ by R. F. Tylecote (1992). The first widespread use began with discovery that Copper (Cu), and later Tin (Sn) could be extracted relatively easily from their ore minerals, giving rise to the ‘Bronze Age’, beginning perhaps 9,000 years ago. It is likely that the discovery of ‘iron’ smelting was accidental, perhaps around 4,700 years ago, and was possibly linked to the use of Iron-rich material in production of copper. By 3,000 years ago, ‘iron’ was important in human societies, being used widely in making weapons.
To produce ferrous metal in quantity, it’s necessary to find a good supply of a suitable starting material – the ore. Fuel is required to break the ore down into elemental Iron, typically by raising it to a very high temperature, away from air. It’s also important to be able to remove a range of impurities from the molten metal. Improvements in smelting technique have long been driven by pressures of the cost of mining and transporting ore and fuel, but also reflect the availability of different types of ore. Since the Second World War a very unusual type of ore, Banded Iron Formation (BIF) has been mined in enormous quantities in Australia, Brazil, the USA and Russia (among other countries). BIF is a very peculiar sedimentary rock, deposited in ancient seas, more than two billion years ago when the atmosphere and oceans had very different behaviour to the modern world. Because it is available in large quantities (many millions of tonnes per annum) and can be processed quite easily to concentrate the content of Iron, it is now most economic to transport this ore worldwide, rather than smelting at source in areas lacking fuel. Before the use of BIF, most production usually relied upon local supplies of ore, as well as coal, coke or charcoal, and additives to help separate metal and slag. In Britain, we have no BIF, and there’s little in Europe as a whole. The history of ferrous metal production in Britain therefore reflects making do with what was available, and many different types of Iron-rich rocks (ironstones) were used as ore.
The most common natural Iron-rich materials found on the modern Earth’s surface are oxide minerals, carbonates, sulfides and fine aluminosilicates. The oxides may be loose mineral grains from weathering of igneous rocks such as basalt lavas, or may form by reaction of volcanic glass and Iron-bearing silicate minerals (such as olivine or pyroxene) with Oxygen and water, especially during tropical weathering. Two minerals are often formed : Goethite (yellow-brown oxyhydroxide, FeO.OH, about 60% Iron by weight) and Hematite (red-purple-grey oxide Fe2O3, nearly 70% Iron by weight), both contain Iron in an oxidised form, Fe3+, which is not very soluble in water. As anyone who has owned an old car will know, metallic ‘iron’ and steel are also able (and all too willing) to form similar oxidised rust! The insoluble oxyhydroxides and oxides are very widespread as tiny grains in soils, giving brown or red colouration. Accumulation in dense soil layers can produce material suitable for use as ore, but these minerals were also occasionally deposited from warm water flowing through cracks in rock, and may form patches and veins of very high grade ores, such as the red Hematite ‘kidney ore’ of Egremont in Cumbria. BIF contain mainly Hematite, in layers with silica.
However, if the tiny grains are washed away by streams and rivers until they reach still water, they can sink and become gently buried within muddy sediment in a lake, delta-front or quiet-water sea. Here they are effectively cut off from air, and as bacterial decay of organic matter in the mud proceeds, they may again lose Oxygen, releasing soluble Fe2+ ions. In freshwater, the ‘reduced’ soluble Iron may react with carbonate created by bacterial oxidation of organic matter (such as rotting leaves), and can be fixed as an insoluble carbonate mineral called Siderite (FeCO3). This often forms spherical concretions that may become flattened as the muddy layers are gradually squashed by continuing build-up of sediment above. The hardened (lithified) concretions or nodules are grey-green when broken, although may turn brown on weathering. Often found in mudstones between coal seams of Carboniferous age across Britain, these Siderite nodules (called ‘doggers’ by miners) may contain nearly 50% Iron by weight, and were an important source of ore during the Industrial Revolution of the seventeenth and eighteenth centuries.
Iron-rich mud deposited in seawater may behave differently. The oxides and oxyhydroxides again release soluble Iron as Fe2+ ions, but bacterial activity near the surface of the accumulating sediment removes Oxygen from the sulfate ions in the seawater, creating sulfide ions. This is how disturbed marine muds often come to smell of ‘rotten eggs’, the characteristic signature of hydrogen sulfide (H2S). Soluble Fe2+ reacts very quickly with sulfide ions, forming a black iron sulfide, and eventually golden Iron Pyrites (FeS2), with about 45% Iron by weight. This can be used as an Iron ore, but releases acidic sulfur dioxide fumes during processing, and requires both careful handling and large amounts of fuel. However, if deposition of mud is quite rapid, the production of sulfide can stop well before all of the soluble Fe has reacted, and more of the carbonate Siderite will then form, often becoming the main Iron-bearing mineral in shallow marine ironstones.
Iron may also be found in pale green hydrated aluminosilicate minerals (containing Al, Si and water), these are members of the Clay Mineral and Chlorite groups, called Berthierine and Chamosite, typically containing about 25% Iron by weight. How these minerals form is still not well understood, despite many studies of ancient and modern ironstones (Kearsley 1989; Young, 1989; Mücke and Farshad, 2005; Clement et al., 2019). There are probably several different origins. Some may be formed by soluble Fe reacting with the white clay mineral Kaolinite within the mud, or from insoluble Fe oxides reacting with Al and Si hydroxides. Some may form by tiny crystals growing within a slimy gelatinous blob or layer, some may grow as crystals directly from water in the mud. Strangely, these minerals also seem to favour growing in layers around a central core, making a concentric tiny egg, an ‘oolith’ or ‘ooid’. When ooids/ooliths are common within an iron-rich rock, it is described as an oolitic ironstone. It is not uncommon to find ironstones that contain aluminosilicates, Siderite, Hematite and Pyrite all together, including within ooliths/ooids – even with evidence that these minerals have replaced each other during or after deposition of the layer.
Oolitic ironstones are complicated rocks (see figure above). As their content of Iron can vary a great deal, they may or may not prove to be an economic source of Iron, which may also depend upon the other materials that they contain. High contents of Calcium (Ca) may help smelting, but high Phosphorus (P) can contaminate the metal that is produced. The oolitic ironstones mined in Rosedale and around all of the North York Moors typically contain mixtures of Siderite and Berthierine, as well as Kaolinite and the Calcium carbonate mineral Calcite.
The oxide Magnetite (Fe3O4) may also be found in some oolitic ironstones, it contains over 70% Iron by weight. As the name suggests, this mineral is strongly magnetic, unlike almost all of the other Iron ore minerals. It is quite common in Mg- and Fe-rich igneous rocks (formed from molten material), and can occur in massive deposits with a very high percentage of Iron. For example, magnetite has long been mined in Sweden, and was much sought after by both Allied and Axis industries during the Second World War. Magnetite is well known to occur in rocks that have been subjected to burial heating (low grade metamorphism), probably growing as coarser crystals from iron carried through porous rock by hot water.
However, it has also been found (and almost completely mined out) in sedimentary ironstone deposits in Rosedale, it was so rich in Iron. Here its origin is still a mystery, and there have been differing interpretations of when and how it formed. There are several 19th century accounts of the discovery of magnetic ores in Rosedale (Bewick 1861; Wood, 1969; Marley 1871), as well as descriptions of these rocks in the Geological Survey Reports of Hallimond (1925) and Whitehead et al. (1952). From other evidence in the North York Moors, it doesn’t seem likely that these rocks were heated sufficiently to encourage metamorphic magnetite replacement of other minerals, and these are definitely not rocks formed from hot melt. Perhaps the peculiar setting where these sedimentary ironstones accumulated was an important factor in creating Magnetite? The earlier accounts suggested that the richest ore was found within elongate troughs, eroded into the underlying layers. Young (1994) suggested that there were indeed shallow basins where ooliths were deposited, but that the basins had been formed by fault motion at about the same time. Is it possible that stagnant water saturating the sediment within these hollows allowed Magnetite to form, replacing other more-oxidised Iron-rich minerals?
Ironstones deposited during the early part of the Jurassic Period have been extensively mined throughout England and Western Scotland, as described in Whitehead et al. (1952). There is a wider discussion of other ironstones from a broader range of ages, across England and Wales, in Hallimond (1925).
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