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bog iron
Bog iron is a form of impure iron deposit that develops in bogs or swamps by the chemical or biochemical oxidation of iron carried in solution. In general, bog ores consist primarily of iron oxyhydroxides, commonly goethite (FeO(OH)).
Iron-bearing groundwater typically emerges as a spring. The iron is oxidized to ferric hydroxide upon encountering the oxidizing environment of the surface. Bog ore often combines goethite, magnetite and vugs or stained quartz. Oxidation may occur through enzyme catalysis by iron bacteria. It is not clear whether the magnetite precipitates upon first contact with oxygen, then oxidizes to ferric compounds, or whether the ferric compounds are reduced when exposed to anoxic conditions upon burial beneath the sediment surface and reoxidized upon exhumation at the surface.
Iron made from bog ore will often contain residual silicates, which can form a glassy coating that grants some resistance to rusting.
== Europe and the North ==
Iron smelting from bog iron was invented during the Pre-Roman Iron Age, and most Viking era iron was smelted from bog iron. The bog iron deposits of Northern and Northeastern Europe were created after the Ice Age ended, on postglacial plains. In Russia, bog ore was the principal source of iron until the 16th century, when the superior ores of Ural Mountains became available.
"Streams carry dissolved iron from nearby mountains. In the bog, the iron is concentrated by two processes. The bog environment is acidic, with a low concentration of dissolved oxygen. In the acidic environment of the bog, a chemical reaction forms insoluble iron compounds which precipitate out. But more importantly, anaerobic bacteria (''Gallionella'' and ''Leptothrix'') growing under the surface of the bog concentrate the iron as part of their life processes...Their presence can be detected on the surface by the iridescent oily film they leave on the water (left), another sure sign of bog iron. In Iceland, the film is called jarnbrák (iron slick). When a layer of peat in the bog is cut and pulled back using turf knives, pea sized nodules of bog iron can be found and harvested. Although the iron nodules are reasonably pure, there aren't many of them. They are, however, a renewable resource. About once each generation, the same bog can be re-harvested."〔(【引用サイトリンク】 Iron Production in the Viking Age )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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