Editing Banded iron formation (section)

==Description==
[[Image:Banded Iron Formation Barberton.jpg|thumb|Banded iron formation from the [[Barberton Greenstone Belt]], South Africa]]
A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black [[iron oxide]]s, either [[magnetite]] (Fe<sub>3</sub>O<sub>4</sub>) or [[hematite]] (Fe<sub>2</sub>O<sub>3</sub>), alternating with bands of iron-poor [[chert]], often red in color, of similar thickness.<ref name="james-1954">{{cite journal |last1=James |first1=Harold Lloyd |title=Sedimentary facies of iron-formation |journal=Economic Geology |date=1 May 1954 |volume=49 |issue=3 |pages=235–293 |doi=10.2113/gsecongeo.49.3.235|bibcode=1954EcGeo..49..235J }}</ref><ref name="trendall-2002">{{cite encyclopedia |last1=Trendall |first1=A.F. |year=2002 |encyclopedia=Precambrian Sedimentary Environments: A Modern Approach to Ancient Depositional Systems |editor1-last=Altermann |editor1-first=Wladyslaw |editor2-last=Corcoran |editor2-first=Patricia L. |isbn=0-632-06415-3 |publisher=Blackwell Science Ltd |pages=33–36 |title=The significance of iron-formation in the Precambrian stratigraphic record}}</ref><ref name=Katsuta2012>{{cite journal | vauthors = Katsuta N, Shimizu I, Helmstaedt H, Takano M, Kawakami S, Kumazawa M |title=Major element distribution in Archean banded iron formation (BIF): influence of metamorphic differentiation |journal=Journal of Metamorphic Geology |date= June 2012 |volume=30 |issue=5 |pages=457–472 |doi=10.1111/j.1525-1314.2012.00975.x |bibcode=2012JMetG..30..457K |s2cid=129322335 }}</ref><ref name="condie-2015">{{cite book |last=Condie |first=Kent C. |author-link=Kent Condie |title=Earth as an evolving planetary system |date=2015 |publisher=Academic Press |isbn=9780128036891 |edition=3}}</ref> A single banded iron formation can be up to several hundred meters in thickness and extend laterally for several hundred kilometers.<ref name="trendall-blockley-2004"/>

Banded iron formation is more precisely defined as chemically precipitated [[sedimentary rock]] containing greater than 15% [[iron]]. However, most BIFs have a higher content of iron, typically around 30% by mass, so that roughly half the rock is iron oxides and the other half is silica.<ref name="trendall-blockley-2004">{{cite encyclopedia |chapter=Precambrian iron-formation |first1=A.F. |last1=Trendall |first2=J.G. |last2=Blockley |title=Evolution of the Hydrosphere and Atmosphere |editor1-first=P.G. |editor1-last=Eriksson |editor2-first=W. |editor2-last=Altermann |editor3-first=D.R. |editor3-last=Nelson |editor4-first=W.U. |editor4-last=Mueller |editor5-first=O. |editor5-last=Catuneanu |encyclopedia=Developments in Precambrian Geology
 |series=Developments in Precambrian Geology |volume=12 |year=2004 |pages=359–511
|doi=10.1016/S0166-2635(04)80007-0|isbn=9780444515063 }}</ref><ref name="trendall-2005">{{cite encyclopedia|last1=Trendall |first1=A. |chapter=Banded iron formations |encyclopedia=Encyclopedia of Geology |year=2005 |pages=37–42 |publisher=Elsevier }}</ref> The iron in BIFs is divided roughly equally between the more oxidized [[ferric]] form, Fe(III), and the more reduced [[ferrous]] form, Fe(II), so that the ratio Fe(III)/Fe(II+III) typically varies from 0.3 to 0.6. This indicates a predominance of magnetite, in which the ratio is 0.67, over hematite, for which the ratio is 1.<ref name="condie-2015"/> In addition to the iron oxides (hematite and magnetite), the iron sediment may contain the iron-rich carbonates [[siderite]] and [[ankerite]], or the iron-rich silicates [[minnesotaite]] and [[greenalite]]. Most BIFs are chemically simple, containing little but iron oxides, silica, and minor carbonate,<ref name="trendall-blockley-2004"/> though some contain significant calcium and magnesium, up to 9% and 6.7% as oxides respectively.<ref name="gole-klein-1981"/><ref name="klein-2005"/>

When used in the singular, the term banded iron formation refers to the sedimentary lithology just described.<ref name="james-1954"/> The plural form, banded iron formations, is used informally to refer to stratigraphic units that consist primarily of banded iron formation.<ref name="explanation">Examples of this usage are found in Gole and Klein 1981; Klein 2005; Trendall 2005; and  Zhu ''et al.'' 2014.</ref>

A well-preserved banded iron formation typically consists of ''macrobands'' several meters thick that are separated by thin [[shale]] beds. The macrobands in turn are composed of characteristic alternating layers of chert and iron oxides, called ''mesobands'', that are several millimeters to a few centimeters thick. Many of the chert mesobands contain ''microbands'' of iron oxides that are less than a millimeter thick, while the iron mesobands are relatively featureless. BIFs tend to be extremely hard, tough, and dense, making them highly resistant to erosion, and they show fine details of stratification over great distances, suggesting they were deposited in a very low-energy environment; that is, in relatively deep water, undisturbed by wave motion or currents.<ref name="trendall-2002"/> BIFs only rarely interfinger with other rock types, tending to form sharply bounded discrete units that never grade laterally into other rock types.<ref name="trendall-blockley-2004"/>

[[Image:MichiganBIF.jpg|thumb|Close-up of banded iron formation specimen from [[Upper Michigan]]]]
Banded iron formations of the [[Great Lakes region]] and the Frere Formation of western [[Australia]] are somewhat different in character and are sometimes described as ''granular iron formations'' or ''GIFs''.<ref name="gole-klein-1981"/><ref name="trendall-blockley-2004"/> Their iron sediments are granular to [[Oolite|oolitic]] in character, forming discrete grains about a millimeter in diameter, and they lack microbanding in their chert mesobands. They also show more irregular mesobanding, with indications of ripples and other [[sedimentary structures]], and their mesobands cannot be traced out any great distance. Though they form well-defined, discrete units, these are commonly interbedded with coarse to medium-grained epiclastic sediments (sediments formed by weathering of rock). These features suggest a higher energy [[depositional environment]], in shallower water disturbed by wave motions. However, they otherwise resemble other banded iron formations.<ref name="gole-klein-1981">{{cite journal |last1=Gole |first1=Martin J. |last2=Klein |first2=Cornelis |title=Banded Iron-Formations through Much of Precambrian Time |journal=The Journal of Geology |date=March 1981 |volume=89 |issue=2 |pages=169–183 |doi=10.1086/628578|bibcode=1981JG.....89..169G |s2cid=140701897 }}</ref>

[[File:Rapitan BIF south australia.jpg|thumb|[[Thin section]] of Neoproterozoic banded iron formation from Australia]]
The great majority of banded iron formations are [[Archean]] or [[Paleoproterozoic]] in age.  However, a small number of BIFs are [[Neoproterozoic]] in age, and are frequently,<ref name="klein-2005"/><ref name="ilyin-2009"/><ref name="bekker-etal-2010">{{cite journal |last1 = Bekker |first1=A |last2=Slack |first2=J.F. |last3=Planavsky |first3=N. |last4=Krapez |first4=B. |last5=Hofmann |first5=A. |last6=Konhauser |first6=K.O. |last7=Rouxel |first7=O.J. | title = Iron formation: the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes. | journal = Economic Geology | date = May 2010 | volume = 105 | issue = 3 | pages = 467–508 | url = http://faculty.eas.ualberta.ca/konhauser/Reprints/EconomicGeology-AB%282010%29.pdf | doi = 10.2113/gsecongeo.105.3.467 |bibcode=2010EcGeo.105..467B | citeseerx = 10.1.1.717.4846 }}</ref> if not universally,<ref name="abd-etal-2019">{{cite journal |last1=Abd El-Rahman |first1=Yasser |last2=Gutzmer |first2=Jens |last3=Li |first3=Xian-Hua |last4=Seifert |first4=Thomas |last5=Li |first5=Chao-Feng |last6=Ling |first6=Xiao-Xiao |last7=Li |first7=Jiao |title=Not all Neoproterozoic iron formations are glaciogenic: Sturtian-aged non-Rapitan exhalative iron formations from the Arabian–Nubian Shield |journal=Mineralium Deposita |date=6 June 2019 |volume=55 |issue=3 |pages=577–596 |doi=10.1007/s00126-019-00898-0|bibcode=2019MinDe..55..577A |s2cid=189829154 }}</ref> associated with glacial deposits, often containing glacial [[dropstones]].<ref name="klein-2005"/> They also tend to show a higher level of oxidation, with hematite prevailing over magnetite,<ref name="ilyin-2009"/> and they typically contain a small amount of phosphate, about 1% by mass.<ref name="ilyin-2009"/> Mesobanding is often poor to nonexistent<ref name="cox-etal-2013">{{cite journal |last1=Cox |first1=Grant M. |last2=Halverson |first2=Galen P. |last3=Minarik |first3=William G. |last4=Le Heron |first4=Daniel P. |last5=Macdonald |first5=Francis A. |last6=Bellefroid |first6=Eric J. |last7=Straus |first7=Justin V. |title=Neoproterozoic iron formation: An evaluation of its temporal, environmental and tectonic significance |journal=Chemical Geology |date=2013 |volume=362 |pages=232–249 |doi=10.1016/j.chemgeo.2013.08.002 |bibcode=2013ChGeo.362..232C |s2cid=56300363 |url=https://francismacdonald.fas.harvard.edu/files/fmacdonald/files/cox_2013_chemgeo_bifs.pdf |access-date=23 June 2020}}</ref> and [[soft-sediment deformation structures]] are common. This suggests very rapid deposition.<ref name="stern-etal-2013"/> However, like the granular iron formations of the Great Lakes, the Neoproterozoic occurrences are widely described as banded iron formations.<ref name="klein-2005">{{cite journal |last1=Klein |first1=C. |title=Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins |journal=American Mineralogist |date=1 October 2005 |volume=90 |issue=10 |pages=1473–1499 |doi=10.2138/am.2005.1871|bibcode=2005AmMin..90.1473K |s2cid=201124189 }}</ref><ref name="ilyin-2009">{{cite journal |last1=Ilyin |first1=A. V. |title=Neoproterozoic banded iron formations |journal=Lithology and Mineral Resources |date=9 January 2009 |volume=44 |issue=1 |pages=78–86 |doi=10.1134/S0024490209010064|s2cid=129978001 }}</ref><ref name="stern-etal-2013">{{cite journal |last1=Stern |first1=Robert J. |last2=Mukherjee |first2=Sumit K. |last3=Miller |first3=Nathan R. |last4=Ali |first4=Kamal |last5=Johnson |first5=Peter R. |title=~750Ma banded iron formation from the Arabian-Nubian Shield—Implications for understanding neoproterozoic tectonics, volcanism, and climate change |journal=Precambrian Research |date=December 2013 |volume=239 |pages=79–94 |doi=10.1016/j.precamres.2013.07.015|bibcode=2013PreR..239...79S }}</ref><ref name="condie-2015"/><ref name="gaucher-etal-2015">{{cite encyclopedia |last1=Gaucher |first1=Cladio |last2=Sial |first2=Alcides N. |last3=Frei |first3=Robert |title=Chapter 17: Chemostratigraphy of Neoproterozoic Banded Iron Formation (BIF): Types, Age and Origin |chapter=Chapter 17 - Chemostratigraphy of Neoproterozoic Banded Iron Formation (BIF): Types, Age and Origin |encyclopedia=Chemostratigraphy: Concepts, Techniques, and Applications |date=2015 |pages=433–449 |doi=10.1016/B978-0-12-419968-2.00017-0 |isbn=9780124199682 |url=https://www.sciencedirect.com/science/article/pii/B9780124199682000170 |access-date=22 June 2020}}</ref><ref name="li-etal-2018">{{cite journal |last1=Li |first1=Zhi-Quan |last2=Zhang |first2=Lian-Chang |last3=Xue |first3=Chun-Ji |last4=Zheng |first4=Meng-Tian |last5=Zhu |first5=Ming-Tian |last6=Robbins |first6=Leslie J. |last7=Slack |first7=John F. |last8=Planavsky |first8=Noah J. |last9=Konhauser |first9=Kurt O. |title=Earth's youngest banded iron formation implies ferruginous conditions in the Early Cambrian ocean |journal=Scientific Reports |date=2 July 2018 |volume=8 |issue=1 |page=9970 |doi=10.1038/s41598-018-28187-2|pmid=29967405 |pmc=6028650 |bibcode=2018NatSR...8.9970L |doi-access=free }}</ref>

Banded iron formations are distinct from most [[Phanerozoic]] [[ironstone]]s. Ironstones are relatively rare and are thought to have been deposited in marine [[anoxic event]]s, in which the depositional basin became depleted in free [[oxygen]]. They are composed of iron silicates and oxides without appreciable chert but with significant [[phosphorus]] content, which is lacking in BIFs.<ref name="bekker-etal-2010"/>

No classification scheme for banded iron formations has gained complete acceptance.<ref name="trendall-blockley-2004"/> In 1954, Harold Lloyd James advocated a classification based on four lithological facies (oxide, carbonate, silicate, and sulfide) assumed to represent different depths of deposition,<ref name="james-1954"/> but this speculative model did not hold up.<ref name="trendall-blockley-2004"/> In 1980, Gordon A. Gross advocated a twofold division of BIFs into an Algoma type and a Lake Superior type, based on the character of the depositional basin. Algoma BIFs are found in relatively small basins in association with [[greywacke]]s and other volcanic rocks and are assumed to be associated with volcanic centers. Lake Superior BIFs are found in larger basins in association with black shales, [[quartzite]]s, and [[Dolomite (rock)|dolomite]]s, with relatively minor [[tuff]]s or other volcanic rocks, and are assumed to have formed on a [[continental shelf]].<ref name="gross-1980">{{cite journal|last1=Gross |first1=G.A. |year=1980 |title=A classification of iron formations based on depositional environments |journal=The Canadian Mineralogist  |volume=18 |pages=215–222}}</ref> This classification has been more widely accepted, but the failure to appreciate that it is strictly based on the characteristics of the depositional basin and not the lithology of the BIF itself has led to confusion, and some geologists have advocated for its abandonment.<ref name="trendall-2002"/><ref name="ohmoto-2004">{{cite encyclopedia |chapter=The Archean atmosphere, hydrosphere, and biosphere |first1=H. |last1=Ohmoto  |title=Evolution of the Hydrosphere and Atmosphere |editor1-first=P.G. |editor1-last=Eriksson |editor2-first=W. |editor2-last=Altermann |editor3-first=D.R. |editor3-last=Nelson |editor4-first=W.U. |editor4-last=Mueller |editor5-first=O. |editor5-last=Catuneanu |encyclopedia=Developments in Precambrian Geology
 |series=Developments in Precambrian Geology |volume=12 |year=2004 |at=5.2
|doi=10.1016/S0166-2635(04)80007-0|isbn=9780444515063 }}</ref> However, the classification into Algoma versus Lake Superior types continues to be used.<ref name="taner-2015">{{cite journal |last1=Taner |first1=Mehmet F. |last2=Chemam |first2=Madjid |title=Algoma-type banded iron formation (BIF), Abitibi Greenstone belt, Quebec, Canada |journal=Ore Geology Reviews |date=October 2015 |volume=70 |pages=31–46 |doi=10.1016/j.oregeorev.2015.03.016|bibcode=2015OGRv...70...31T |doi-access=free }}</ref><ref name="Precambrian Research 2016 pp. 47–79">{{cite journal | title=Depositional setting of Algoma-type banded iron formation | journal=Precambrian Research | volume=281 | date=2016-08-01 | issn=0301-9268 | doi=10.1016/j.precamres.2016.04.019 | pages=47–79 | last1=Gourcerol | first1=B. | last2=Thurston | first2=P.C. | last3=Kontak | first3=D.J. | last4=Côté-Mantha | first4=O. | last5=Biczok | first5=J. | bibcode=2016PreR..281...47G | url=https://hal-brgm.archives-ouvertes.fr/hal-02283951/file/proof.pdf }}</ref>