沼泽铁是不纯的铁在沼泽或湿地的溶液，受到化学或生化氧化而淤积。一般情况下，沼泽矿石的主要成分包括氢氧化铁，通常是针铁矿（FeO(OH)）。

含铁的地下水通常会随着泉水涌出。这些铁遇到表面的氧化环境会成为氢氧化铁。沼泽矿经常结合针铁矿、磁铁矿和孔洞或染色的石英。氧化可能经由铁细菌的酶促反应发生。还不清楚是否磁铁矿第一次接触氧气促成氧化，然后氧化成为三价的铁（三氧化二铁），还是三价的铁曝露在缺氧的环境中，被埋在下面的沉积物表面，并且提升到表面再被氧化。

沼泽矿制成的铁经常包含残余的硅酸盐，可以形成阻止生锈的玻璃状涂层。

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.