User:MinorProphet/Draft subpages/Iron Ores &c.

Origins of iron ores

Iron ore (any commercially valuable iron-bearing mineral) is found in almost all types of rock, either deep underground or on the surface, combined with more or less oxygen, or water, or other elements, as a result of almost any geological process. For example, the geology of north-west England (Lancashire, some now Cumbria) is exceptionally complex, leading to a wide variety of different ores being found within a few miles of each other, in totally different strata and for totally different reasons. On the other hand, the sedimentary ores of Minnesota extend for hundreds of square miles in almost unvarying homegeneous beds, laid down during a single geological age. Really?.

NB! There is probably still much uncertainty about the exact origins of some ores, and these are merely the basics of some theories.

1) Formed deep underground by fiery processes - forming iron oxides only - two types:

A) Igneous

i) in basaltic rocks - gabbros (ie coarse-grained dolerite) - mostly magnetite, often found with titanium

ii) in granitic rocks - these are felsic, more recent, and less dense than basalt, which tends to sink into the Earth's mantle. When formed at great depth, granitic ores only contain magnetite: but in ores formed at moderate depth, any of the hematite-magnetite oxide series can be formed. (Iron Ores of Europe)

Igneous rocks may be some of the most recent - eg Basalt in N. Ireland from the Lower Miocene]

More recent upheavals of the earth can expose deeply-buried ores on the surface (eg high-grade ore in Cumbria but difficult of extraction); on the other hand, where there has been little change over millennia the ore may be of low yield but easy to mine (Minnesota etc. see next.)

2) Formed near, or at the earth's surface - by sedimentary or metamorphic processes (eg magnetite skarn) - forming oxides, hydroxides, and a carbonate. Iron also forms sulphates and complex silicates, but they generally are not suitable for iron and steel making.

B) Sedimentary

Elemental iron from stars dissolved in the oceans with little oxygen present, until photosynthesising organisms arrived. See also Snowball Earth theory. With more oxygen present in the oceans, iron oxide then formed, falling as sediment to the sea bed. This gives rise to banded iron formations, eg taconite (low-yield magnetite) in USA, and channel iron deposits in Oz.

C) Metamorphic

Some sedimentary rocks may have some elements replaced through the action of artesian or surface water. Iron combined with dissolved CO2 in terrestrial water to form iron carbonate (siderite). This is swiftly oxidised into ferric hydrate (goethite, limonite etc.) unless there is an abundance of reducing organic substances (ie plant matter). Phosphoric acid accumulates in plants and turns into ammonium phosphate when they decay. During the sedimentation of the iron ores the phosphoric acid was precipitated as iron phosphate or lime (calcium carbonate) phosphate. By regional metamorphism (pressure, moisture and heat) the iron carbonate and ferric hydrate was finally altered into magnetite and hematite. See section "Origin of Iron Ore Deposits in Crystalline Schists", Nature of Iron Ore Deposits Vol 1.

Manganese is precipitated from solutions in the same way, but after the iron (which has a greater affinity for oxygen): thus manganese ores are [often] to be found above iron ores when less carbonic acid (dissolved CO2) remained in the solution. [However, manganese oxide may be leached downwards through the action of water or other chemicals, and ends up in the iron ore below.]

Kendall shows how FeCl (iron chloride) reacts with limestone to make hematite and calcium chloride, with CO2 given off.

Types of iron ores suitable for steel making

1) Oxides

• Magnetite (Fe3O4) - cubic (Isometric) 72.4% iron [More correctly FeO.Fe2 O3 (formerly called peroxide of iron)]
• Red Hematite (Fe2O3) - hexagonal 70 % iron (Specular ore) Red, blue, micaceous, grey, or black banded. eg Huge Brazilian hematite mine at Cajaras? (formerly called protoxide of iron)
• Martite (Fe2O3) - cubic (isometric) 70% iron (pseudomorph of hematite)

2) Hydroxides

Iron also combines with varying amounts of OH ions or H2O. There are many various chemical combinations (or admixtures) of iron, oxygen, hydrogen and water (formerly called e.g. ferric hydrate or sesquioxide of iron); the terms listed here are among the most frequently used:

• Goethite FeO(OH) 69.2% iron (crystalline)
• Limonite (or brown hematite) Fe2(OH)6 or H2O.Fe2O3 59.8% iron (amorphous) (Brown hematite) (formerly called sesquioxide of iron)
• Turgite Fe4O5 (OH)2 66.2% iron. Can be considered as a mixture (or variety) of hematite and goethite, OR as hydrous hematite 2Fe2O3.H2O

3) Carbonate

• Siderite FeCO3 Up to 62.1% (formerly called iron protoxide), up to 48.27% metallic iron but usually from 15% - 35% Found in chalk or limestone. Also known as Argillaceous Ironstone. Purer varieties are described as spathic ore eg Sussex in olden times???? Amorphous argillaceous (ie clayey) carbonate of the Coal Measures (ie the Carboniferous) is called clay ironstone or clayband. When impregnated with carbonaceous or bituminousmatter, is called blackband (Ores of GB, Geological Survey p. 16) (Scotland) Put the other way round, blackband is coal impregnated with iron carbonate. Ores with around 30% of 'carbonic acid' are obviously carbonates.

Other iron compounds unsuitable for steel-making

Iron also forms other oxides, including simple sulphides (pyrites) and complex silicates, but these are generally not suitable as iron ore because the high content of sulphur or other contaminants makes them uneconomic. However, with careful selection of a combination of ores, the complex reactions in a blast furnace can turn these to advantage and either the impurities come out with the slag, or are incorporated into the finished product.

4) Oxides

• Wustite (FeO) - Iron also makes a simple monoxide but being unstable it is not found in nature: it oxidises quickly, and also forms more complex compounds such as FeCO3 (carbonate), sulphate (FeSO4, pyrites) and silicates (2Fe,SiO3).
• Ilmenite (TiFeO3) (Titaniferous ore) - hexagonal - variable amts. of iron - main source of titanium, extracted by chlorination, not valuable nowadays as iron source(?). Little in Europe, lots in Adirondacks and S.America. Modern blast furnaces cannot use iron ores containing more than 2% TiO2, but in the old bloomery process TiO2 combined with iron oxides and silica to produce highly fluid slags that separate cleanly from iron metal.
• Ulvöspinel (TiFe)2O4 - Only identified in 1943.Titaniferous ore - see next Aha, it tends to oxidise to Magnetite and Ilmenite during subsolidus cooling, forming lamellae in exsolution.

5) Sulphides

• Pyrrhotite Fe6S7 - Fe11S12 - 60-61.6% - Hexagonal (ie Fe n S n+1)
• Pyrite FeS2 46.7% iron - Isometric
• Marcasite FeS2 46.6% iron - Orthorhombic

(all figures from Crowell, Benedict & Murray, C.B. (1911) Iron ores of Lake Superior. Cleveland Ohio: Penton Pub Co., pp. 18-20, except Brazil)

6) Complex silicates

• Chamosite / Thuringite - Hydrous aluminium silicates of iron, which is produced in an environment of low to moderate grade of metamorphosed iron deposits, as gray or black crystals in oolitic iron ore.
• Glauconite - an iron potassium phyllosilicate (mica group) mineral, green in colour.

Concentration of iron in deposits

The amount of metallic iron can vary tremendously in a given location.

References

Notes

Citations

Bibliography

• Beck, Richard (1909). The Nature of Ore Deposits (two vols. in one). Translated by Walter Harvey Weed. New York, London: Hill Publishing Company.

• Crowell, Benedict; Murray, C. B. (1917). The Iron Ores of Lake Superior (3rd ed.). Cleveland, Ohio: Penton Publishing Co. pp. 18–20.

• Greenwood, William Henry; Sexton, A. Humboldt (1907). Iron: Its Sources, Properties and Manufacture. London, Paris, New York, Toronto & Melbourne: Cassell and Company.

• Kendall, J. D. (1893). The Iron Ores of Great Britain and Ireland. London: Crosby Lockwood and Son.

• Roesler, Max (1921). The Iron-Ore Resources of Europe (PDF). Washington: Government Printing Office.

• Smyth, Warrington W.; Percy, John (1856–1861). The Iron Ores of Great Britain, Parts I-IV. Memoirs of the Geological Survey of Britain and of the Museum of Practical Geology. London: HMSO; Longman, Brown, Green and Longmans.{{cite book}}: CS1 maint: date format (link)

Vol. I - The iron ores of the north and north-midland counties of England (1856)

Vol. II - The iron ores of south Staffordshire [pdf 127] (1858)

Vol. III - The iron ores of south Wales [pdf 179 or 183] (1861)

Vol. IV - Iron ores of the Shropshire coalfield p. 237 [pdf 259] (n.d.)