Järnnoduler

Järnnoduler, or iron nodules, are spherical or ellipsoidal concretions composed primarily of iron oxides and hydroxides that form in soils, sedimentary rocks and groundwater-deposited sediments. They range from a few millimeters to several centimeters in size, and are typically hard and rust-colored on weathered surfaces. Their interiors can be dense or show concentric layering around a nucleus such as organic matter or a mineral grain.

Formation occurs where iron-rich porewater or groundwater precipitates iron minerals as redox conditions shift, or where microbial processes promote oxidation of ferrous iron to ferric forms. In soils, cycles of saturation and drying, together with organic matter, promote nucleation and growth, producing nodules that may enclose plant fragments or fossil material. Diagenetic processes in sediments can also contribute to their development.

Mineralogy is usually dominated by iron oxides and oxyhydroxides, such as goethite and hematite, with possible minor amounts of ferrihydrite, clays, carbonates or silicates. In some environments nodules form as part of longer diagenetic sequences and may grow around older grains or fossil debris.

Occurrence is widespread in temperate and tropical soils, as well as in river, lake and coastal sedimentary sequences. In soils, järnnoduler can affect drainage, root penetration and color development, and their presence provides clues to past redox conditions and groundwater movement.

Economic relevance is generally limited; iron nodules themselves are not typically mined as ore. However, larger manganese-iron nodules in marine environments have attracted interest for metal resources. In soil science, järnnoduler are valued as indicators of palaeoredox conditions and soil-forming processes.