In the Earth sciences , aggregate has three possible meanings.
68-397: Jasper , an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases, is an opaque , impure variety of silica , usually red, yellow, brown or green in color; and rarely blue. The common red color is due to iron(III) inclusions . Jasper breaks with a smooth surface and is used for ornamentation or as a gemstone . It can be highly polished and
136-538: A continental shelf . 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. However, the classification into Algoma versus Lake Superior types continues to be used. Banded iron formations are almost exclusively Precambrian in age, with most deposits dating to
204-445: A photic zone inhabited by cyanobacteria that had evolved the capacity to carry out oxygen-producing photosynthesis, but which had not yet evolved enzymes (such as superoxide dismutase ) for living in an oxygenated environment. Such organisms would have been protected from their own oxygen waste through its rapid removal via the reservoir of reduced ferrous iron, Fe(II), in the early ocean. The oxygen released by photosynthesis oxidized
272-434: A tsunami at least 1,000 m (3,300 ft) high at the point of impact, and 100 m (330 ft) high about 3,000 km (1,900 mi) away. It has been suggested that the immense waves and large underwater landslides triggered by the impact caused the mixing of a previously stratified ocean, oxygenated the deep ocean, and ended BIF deposition shortly after the impact. Although Cloud argued that microbial activity
340-514: A Snowball Earth state the continents, and possibly seas at low latitudes, were subject to a severe ice age circa 750 to 580 Ma that nearly or totally depleted free oxygen. Dissolved iron then accumulated in the oxygen-poor oceans (possibly from seafloor hydrothermal vents). Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron. Banded iron formations of this period are predominantly associated with
408-512: A brown Egyptian or red African. Jasper is the main component in the silica-rich parts of banded iron formations (BIFs) which indicate low, but present, amounts of dissolved oxygen in the water such as during the Great Oxidation Event or snowball earths. The red bands are microcrystalline red chert, also called jasper. Picture jaspers exhibit combinations of patterns resulting in what appear to be scenes or images, when seen on
476-404: A challenge. Terms attributed to various well-defined materials includes the geographic locality where it is found, sometimes quite restricted such as "Bruneau" (a canyon) and "Lahontan" (a lake), rivers and even individual mountains; many are fanciful, such as "forest fire" or "rainbow", while others are descriptive, such as "autumn" or "porcelain". A few are designated by the place of origin such as
544-422: A cut section. Such patterns include banding from flow or depositional patterns (from water or wind), as well as dendritic or color variations. Diffusion from a center produces a distinctive orbicular appearance, i.e., leopard skin jasper or linear banding from a fracture as seen in liesegang jasper. Healed, fragmented rock produces brecciated (broken) jasper. While these "picture jaspers" can be found all over
612-532: A deposit at Ettutkan Mountain, Staryi Sibay , Bashkortostan , Russia. (The town of Sibay, in the far south of the Ural Mountains , near the border with Kazakhstan , is noted for its colossal, open-cast copper mine.) Basanite is a deep velvety-black variety of amorphous quartz, of a slightly tougher and finer grain than jasper, and less splintery than hornstone. It was the Lydian stone or touchstone of
680-509: A factor of 50 under conditions of low oxygen. Oxygenic photosynthesis is not the only biogenic mechanism for deposition of banded iron formations. Some geochemists have suggested that banded iron formations could form by direct oxidation of iron by microbial anoxygenic phototrophs . The concentrations of phosphorus and trace metals in BIFs are consistent with precipitation through the activities of iron-oxidizing bacteria. Iron isotope ratios in
748-657: 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. BIFs only rarely interfinger with other rock types, tending to form sharply bounded discrete units that never grade laterally into other rock types. Banded iron formations of
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#1732772650815816-447: A higher energy depositional environment , in shallower water disturbed by wave motions. However, they otherwise resemble other banded iron formations. 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, if not universally, associated with glacial deposits, often containing glacial dropstones . They also tend to show
884-734: A higher level of oxidation, with hematite prevailing over magnetite, and they typically contain a small amount of phosphate, about 1% by mass. Mesobanding is often poor to nonexistent and soft-sediment deformation structures are common. This suggests very rapid deposition. However, like the granular iron formations of the Great Lakes, the Neoproterozoic occurrences are widely described as banded iron formations. Banded iron formations are distinct from most Phanerozoic ironstones . Ironstones are relatively rare and are thought to have been deposited in marine anoxic events , in which
952-541: A hydrous silica gel. The conversion of iron hydroxide and silica gels to banded iron formation is an example of diagenesis , the conversion of sediments into solid rock. There is evidence that banded iron formations formed from sediments with nearly the same chemical composition as is found in the BIFs today. The BIFs of the Hamersley Range show great chemical homogeneity and lateral uniformity, with no indication of any precursor rock that might have been altered to
1020-430: A key element of most theories of deposition. The few formations deposited after 1,800 Ma may point to intermittent low levels of free atmospheric oxygen, while the small peak at 750 million years ago may be associated with the hypothetical Snowball Earth. The microbands within chert layers are most likely varves produced by annual variations in oxygen production. Diurnal microbanding would require
1088-625: A mode of formation does not require a global anoxic ocean, but is consistent with either a Snowball Earth or Slushball Earth model. Banded iron formations provide most of the iron ore presently mined. More than 60% of global iron reserves are in the form of banded iron formation, most of which can be found in Australia, Brazil, Canada, India, Russia, South Africa, Ukraine, and the United States. Different mining districts coined their own names for BIFs. The term "banded iron formation"
1156-609: A number of localities. The "Lydian Stone" known to the Ancient Greeks is named for the ancient kingdom of Lydia in what is now western Turkey . A similar rock type occurs in New England . Such rock types have long been used for the making of touchstones to test the purity of precious metal alloys , because they are hard enough to scratch such metals, which, if drawn (scraped) across them, show to advantage their metallic streaks of various (diagnostic) colours, against
1224-472: A peculiar kind of Precambrian evaporite . Other proposed abiogenic processes include radiolysis by the radioactive isotope of potassium , K, or annual turnover of basin water combined with upwelling of iron-rich water in a stratified ocean. Another abiogenic mechanism is photooxidation of iron by sunlight. Laboratory experiments suggest that this could produce a sufficiently high deposition rate under likely conditions of pH and sunlight. However, if
1292-408: A process that did not produce great quantities of biomass, so that little carbon was present to reduce hematite to magnetite. However, it is possible that BIF was altered from carbonate rock or from hydrothermal mud during late stages of diagenesis. A 2018 study found no evidence that magnetite in BIF formed by decarbonization, and suggests that it formed from thermal decomposition of siderite via
1360-827: A thickness of 60 meters (200 feet). Other examples of early Archean BIFs are found in the Abitibi greenstone belts , the greenstone belts of the Yilgarn and Pilbara cratons , the Baltic shield , and the cratons of the Amazon , north China , and south and west Africa. The most extensive banded iron formations belong to what A.F. Trendall calls the Great Gondwana BIFs. These are late Archean in age and are not associated with greenstone belts. They are relatively undeformed and form extensive topographic plateaus, such as
1428-497: A thin layer on the ocean floor. Each band is similar to a varve , resulting from cyclic variations in oxygen production. Banded iron formations were first discovered in northern Michigan in 1844. Banded iron formations account for more than 60% of global iron reserves and provide most of the iron ore presently mined. Most formations can be found in Australia , Brazil , Canada , India , Russia , South Africa , Ukraine , and
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#17327726508151496-485: 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 greywackes 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, quartzites , and dolomites , with relatively minor tuffs or other volcanic rocks, and are assumed to have formed on
1564-429: A very high rate of deposition of 2 meters per year or 5 km/Ma. Estimates of deposition rate based on various models of deposition and sensitive high-resolution ion microprobe (SHRIMP) estimates of the age of associated tuff beds suggest a deposition rate in typical BIFs of 19 to 270 m/Ma, which are consistent either with annual varves or rhythmites produced by tidal cycles. Preston Cloud proposed that mesobanding
1632-458: Is a mass of mineral crystals, mineraloid particles or rock particles. Examples are dolomite , which is an aggregate of crystals of the mineral dolomite , and rock gypsum , an aggregate of crystals of the mineral gypsum . Lapis lazuli is a type of rock composed of an aggregate of crystals of many minerals including lazurite , pyrite , phlogopite , calcite , potassium feldspar , wollastonite and some sodalite group minerals. In
1700-668: Is believed to be unrelated to that of the English given name Jasper , which is of Persian origin, though the Persian word for the mineral jasper is also yashp ( یَشم ). Green jasper was used to make bow drills in Mehrgarh between 4th and 5th millennium BC. Jasper is known to have been a favorite gem in the ancient world; its name can be traced back in Arabic , Persian, Hebrew, Assyrian, Greek and Latin . On Minoan Crete , jasper
1768-411: 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. 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
1836-425: Is oxidation by anaerobic denitrifying bacteria . This requires that nitrogen fixation by microorganisms is also active. The lack of organic carbon in banded iron formation argues against microbial control of BIF deposition. On the other hand, there is fossil evidence for abundant photosynthesizing cyanobacteria at the start of BIF deposition and of hydrocarbon markers in shales within banded iron formation of
1904-544: Is used for items such as vases, seals , and snuff boxes . The density of jasper is typically 2.5 to 2.9 g/cm. Jaspillite is a banded-iron-formation rock that often has distinctive bands of jasper. The name means "spotted or speckled stone," and is derived via Old French jaspre (variant of Anglo-Norman jaspe ) and Latin iaspidem (nom. iaspis ) from Greek ἴασπις iaspis (feminine noun), from an Afroasiatic language (cf. Hebrew ישפה yashpeh , Akkadian yashupu ). This Semitic etymology
1972-529: The Nibelungenlied as being clear and green. The jasper of the ancients probably included stones which would now be classed as chalcedony , and the emerald-like jasper may have been akin to the modern chrysoprase . The Hebrew word may have designated a green jasper. Flinders Petrie suggested that the odem – the first stone on the High Priest's breastplate – was a red jasper, whilst tarshish ,
2040-834: 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 . Their iron sediments are granular to 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
2108-797: The Hamersley Range . The banded iron formations here were deposited from 2470 to 2450 Ma and are the thickest and most extensive in the world, with a maximum thickness in excess of 900 meters (3,000 feet). Similar BIFs are found in the Carajás Formation of the Amazon craton, the Cauê Itabirite of the São Francisco craton , the Kuruman Iron Formation and Penge Iron Formation of South Africa, and
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2176-984: The Mulaingiri Formation of India . Paleoproterozoic banded iron formations are found in the Iron Range and other parts of the Canadian Shield . The Iron Range is a group of four major deposits: the Mesabi Range , the Vermilion Range , the Gunflint Range , and the Cuyuna Range . All are part of the Animikie Group and were deposited between 2500 and 1800 Ma. These BIFs are predominantly granular iron formations. Neoproterozoic banded iron formations include
2244-575: The Sturtian glaciation . An alternative mechanism for banded iron formations in the Snowball Earth era suggests the iron was deposited from metal-rich brines in the vicinity of hydrothermally active rift zones due to glacially-driven thermal overturn. The limited extent of these BIFs compared with the associated glacial deposits, their association with volcanic formations, and variation in thickness and facies favor this hypothesis. Such
2312-553: The United States . A typical banded iron formation consists of repeated, thin layers (a few millimeters to a few centimeters in thickness) of silver to black iron oxides , either magnetite (Fe 3 O 4 ) or hematite (Fe 2 O 3 ), alternating with bands of iron-poor chert , often red in color, of similar thickness. A single banded iron formation can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Banded iron formation
2380-408: The construction industry, an aggregate (often referred to as a construction aggregate ) is sand , gravel or crushed rock that has been mined or quarried for use as a building material . In pedology , an aggregate is a mass of soil particles. If the aggregate has formed naturally, it can be called a ped ; if formed artificially, it can be called a clod. Aggregates are used extensively in
2448-452: The oxygenation of the Earth's oceans . Some of the Earth's oldest rock formations, which formed about 3,700 million years ago ( Ma ), are associated with banded iron formations. Banded iron formations are thought to have formed in sea water as the result of oxygen production by photosynthetic cyanobacteria . The oxygen combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming
2516-592: The Archean. These older BIFs tend to show a positive europium anomaly consistent with a hydrothermal source of iron. By contrast, Lake Superior-type banded iron formations primarily formed during the Paleoproterozoic era, and lack the europium anomalies of the older Algoma-type BIFs, suggesting a much greater input of iron weathered from continents. The absence of hydrogen sulfide in anoxic ocean water can be explained either by reduced sulfur flux into
2584-458: The Fe(II) to ferric iron, Fe(III), which precipitated out of the sea water as insoluble iron oxides that settled to the ocean floor. Cloud suggested that banding resulted from fluctuations in the population of cyanobacteria due to free radical damage by oxygen. This also explained the relatively limited extent of early Archean deposits. The great peak in BIF deposition at the end of the Archean
2652-528: The Great Oxygenation Event. Prior to 2.45 billion years ago, the high degree of mass-independent fractionation of sulfur (MIF-S) indicates an extremely oxygen-poor atmosphere. The peak of banded iron formation deposition coincides with the disappearance of the MIF-S signal, which is interpreted as the permanent appearance of oxygen in the atmosphere between 2.41 and 2.35 billion years ago. This
2720-486: The Pilbara craton. The carbon that is present in banded iron formations is enriched in the light isotope, C, an indicator of a biological origin. If a substantial part of the original iron oxides was in the form of hematite, then any carbon in the sediments might have been oxidized by the decarbonization reaction: Trendall and J.G. Blockley proposed, but later rejected, the hypothesis that banded iron formation might be
2788-585: The Precambrian world, they have been intensively studied by geologists. Banded iron formations are found worldwide, in every continental shield of every continent. The oldest BIFs are associated with greenstone belts and include the BIFs of the Isua Greenstone Belt , the oldest known, which have an estimated age of 3700 to 3800 Ma. The Temagami banded iron deposits formed over a 50-million-year period, from 2736 to 2687 Ma, and reached
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2856-631: The Urucum in Brazil, Rapitan in the Yukon , and the Damara Belt in southern Africa. They are relatively limited in size, with horizontal extents not more than a few tens of kilometers and thicknesses not more than about 10 meters (33 feet). These are widely thought to have been deposited under unusual anoxic oceanic conditions associated with the " Snowball Earth ." Banded iron formation provided some of
2924-670: The ancients. It is mentioned and its use described in the writings of Bacchylides about 450 BC, and was also described by Theophrastus in his book On Stones ( Ancient Greek title: Περὶ λίθων : Peri Lithon ), a century later. It is evident that the touchstone that Pliny had in mind when he wrote about it was merely a dense variety of basalt . Basanite (not to be confused with bassanite ), Lydian stone , and radiolarite (a.k.a. lydite or flinty slate) are terms used to refer to several types of black, jasper-like rock (also including tuffs , cherts and siltstones ) which are dense, fine-grained and flinty / cherty in texture and found in
2992-424: The availability of reduced iron on time scales of decades. In the case of granular iron formations, the mesobands are attributed to winnowing of sediments in shallow water, in which wave action tended to segregate particles of different size and composition. For banded iron formations to be deposited, several preconditions must be met. There must be an ample source of reduced iron that can circulate freely into
3060-662: The construction industry Often in making concrete , a construction aggregate is used, with about 6 billion tons of concrete produced per year. This mineralogy article is a stub . You can help Misplaced Pages by expanding it . Banded iron formation Banded iron formations ( BIFs ; also called banded ironstone formations ) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert . They can be up to several hundred meters in thickness and extend laterally for several hundred kilometers. Almost all of these formations are of Precambrian age and are thought to record
3128-542: The current composition. This suggests that, other than dehydration and decarbonization of the original ferric hydroxide and silica gels, diagenesis likely left the composition unaltered and consisted of crystallization of the original gels. Decarbonization may account for the lack of carbon and preponderance of magnetite in older banded iron formations. The relatively high content of hematite in Neoproterozoic BIFs suggests they were deposited very quickly and via
3196-549: The dark background. There are, confusingly, not one but two rocks called basanite, one being a black form of jasper and the other a black volcanic rock closely akin to basalt. Add to this the fact that many different rock types – having in common the colour black and a fine texture – have, over the ages, been pressed into service as touchstones and it will be seen that there is ample scope for confusion in this petrology - and mineralogy -related field of study. Aggregate (geology) In mineralogy and petrology , an aggregate
3264-417: The deep ocean became sufficiently oxygenated at that time to end transport of reduced iron. Heinrich Holland argues that the absence of manganese deposits during the pause between Paleoproterozoic and Neoproterozoic BIFs is evidence that the deep ocean had become at least slightly oxygenated. The "Canfield ocean" model proposes that, to the contrary, the deep ocean became euxinic and transport of reduced iron
3332-472: The deep ocean or a lack of dissimilatory sulfate reduction (DSR), the process by which microorganisms use sulfate in place of oxygen for respiration. The product of DSR is hydrogen sulfide, which readily precipitates iron out of solution as pyrite. The requirement of an anoxic, but not euxinic, deep ocean for deposition of banded iron formation suggests two models to explain the end of BIF deposition 1.8 billion years ago. The "Holland ocean" model proposes that
3400-412: The deposition basin. Plausible sources of iron include hydrothermal vents along mid-ocean ridges, windblown dust, rivers, glacial ice, and seepage from continental margins. The importance of various sources of reduced iron has likely changed dramatically across geologic time. This is reflected in the division of BIFs into Algoma and Lake Superior-type deposits. Algoma-type BIFs formed primarily in
3468-551: 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. No classification scheme for banded iron formations has gained complete acceptance. 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, but this speculative model did not hold up. In 1980, Gordon A. Gross advocated
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#17327726508153536-404: The diffusion of minerals along discontinuities providing the appearance of vegetative growth, i.e., dendritic . The original materials are often fractured and/or distorted, after deposition, into diverse patterns, which are later filled in with other colorful minerals. Weathering, with time, will create intensely colored superficial rinds. The classification and naming of jasper varieties presents
3604-429: The first evidence for the timing of the Great Oxidation Event , 2,400 Ma. With his 1968 paper on the early atmosphere and oceans of the Earth, Preston Cloud established the general framework that has been widely, if not universally, accepted for understanding the deposition of BIFs. Cloud postulated that banded iron formations were a consequence of anoxic, iron-rich waters from the deep ocean welling up into
3672-442: The iron came from a shallow hydrothermal source, other laboratory experiments suggest that precipitation of ferrous iron as carbonates or silicates could seriously compete with photooxidation. Regardless of the precise mechanism of oxidation, the oxidation of ferrous to ferric iron likely caused the iron to precipitate out as a ferric hydroxide gel. Similarly, the silica component of the banded iron formations likely precipitated as
3740-566: The late Archean (2800–2500 Ma) with a secondary peak of deposition in the Orosirian period of the Paleoproterozoic (1850 Ma). Minor amounts were deposited in the early Archean and in the Neoproterozoic (750 Ma). The youngest known banded iron formation is an Early Cambrian formation in western China. Because the processes by which BIFs are formed appear to be restricted to early geologic time, and may reflect unique conditions of
3808-441: The late Archean peak of BIF deposition was spread out over tens of millions of years, rather than taking place in a very short interval of time following the evolution of oxygen-coping mechanisms. However, his general concepts continue to shape thinking about the origins of banded iron formations. In particular, the concept of the upwelling of deep ocean water, rich in reduced iron, into an oxygenated surface layer poor in iron remains
3876-592: The oldest banded iron formations (3700-3800 Ma), at Isua, Greenland, are best explained by assuming extremely low oxygen levels (<0.001% of modern O 2 levels in the photic zone) and anoxygenic photosynthetic oxidation of Fe(II): This requires that dissimilatory iron reduction, the biological process in which microorganisms substitute Fe(III) for oxygen in respiration, was not yet widespread. By contrast, Lake Superior-type banded iron formations show iron isotope ratios that suggest that dissimilatory iron reduction expanded greatly during this period. An alternate route
3944-587: 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. 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, though some contain significant calcium and magnesium, up to 9% and 6.7% as oxides respectively. When used in
4012-505: The reaction The iron may have originally precipitated as greenalite and other iron silicates. Macrobanding is then interpreted as a product of compaction of the original iron silicate mud. This produced siderite-rich bands that served as pathways for fluid flow and formation of magnetite. The peak of deposition of banded iron formations in the late Archean, and the end of deposition in the Orosirian, have been interpreted as markers for
4080-528: The singular, the term banded iron formation refers to the sedimentary lithology just described. The plural form, banded iron formations, is used informally to refer to stratigraphic units that consist primarily of banded iron formation. 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
4148-433: The tenth stone, may have been a yellow jasper. Jasper is an opaque rock of virtually any colour stemming from the mineral content of the original sediments or ash. Patterns arise during the consolidation process forming flow and depositional patterns in the original silica-rich sediment or volcanic ash . Hydrothermal circulation is generally thought to be required in the formation of jasper. Jasper can be modified by
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#17327726508154216-622: The world, specific colors or patterns are unique to the geographic region from which they originate. One source of the stone is Indonesia , especially in Purbalingga district. From the US, Oregon 's Biggs jasper and Idaho 's Bruneau jasper from the Bruneau River canyon are particularly fine examples. Other examples can be seen at Ynys Llanddwyn in Wales . A blue-green jasper occurs in
4284-529: Was a key process in the deposition of banded iron formation, the role of oxygenic versus anoxygenic photosynthesis continues to be debated, and nonbiogenic processes have also been proposed. Cloud's original hypothesis was that ferrous iron was oxidized in a straightforward manner by molecular oxygen present in the water: The oxygen comes from the photosynthetic activities of cyanobacteria. Oxidation of ferrous iron may have been hastened by aerobic iron-oxidizing bacteria, which can increase rates of oxidation by
4352-406: Was a result of self-poisoning by early cyanobacteria as the supply of reduced iron was periodically depleted. Mesobanding has also been interpreted as a secondary structure, not present in the sediments as originally laid down, but produced during compaction of the sediments. Another theory is that mesobands are primary structures resulting from pulses of activity along mid-ocean ridges that change
4420-565: Was accompanied by the development of a stratified ocean with a deep anoxic layer and a shallow oxidized layer. The end of deposition of BIF at 1.85 billion years ago is attributed to the oxidation of the deep ocean. Until 1992 it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally. Iron-rich waters would then form in isolation and subsequently come into contact with oxygenated water. The Snowball Earth hypothesis provided an alternative explanation for these younger deposits. In
4488-479: Was blocked by precipitation as pyrite. Banded iron formations in northern Minnesota are overlain by a thick layer of ejecta from the Sudbury Basin impact. An asteroid (estimated at 10 km (6.2 mi) across) impacted into waters about 1,000 m (3,300 ft) deep 1.849 billion years ago, coincident with the pause in BIF deposition. Computer models suggest that the impact would have generated
4556-412: Was carved to produce seals circa 1800 BC, as evidenced by archaeological recoveries at the palace of Knossos . Although the term jasper is now restricted to opaque quartz, the ancient iaspis was a stone of considerable translucency including nephrite . The jasper of antiquity was in many cases distinctly green, for it is often compared to emerald and other green objects. Jasper is referred to in
4624-432: Was thought to be the result of the evolution of mechanisms for living with oxygen. This ended self-poisoning and produced a population explosion in the cyanobacteria that rapidly depleted the remaining supply of reduced iron and ended most BIF deposition. Oxygen then began to accumulate in the atmosphere. Some details of Cloud's original model were abandoned. For example, improved dating of Precambrian strata has shown that
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