concretions

Concretions are compact masses of mineral matter that form by precipitation of minerals around a central nucleus within sedimentary rocks or soils. They commonly differ in composition and hardness from the surrounding matrix and are often more resistant to weathering, so they can remain as rounded, isolated bodies after erosion.

Formation occurs as mineral-rich fluids move through sediments and precipitate minerals around an available nucleus, such as a fossil fragment, shell, plant particle, or a grain of sediment. Growth is typically radial, producing spherical, oval, or irregular nodules. The cementing mineral varies by environment and may include calcite, siderite, silica, or iron oxides or sulfides.

Common types include carbonate concretions (calcite or aragonite), iron oxide or iron sulfide concretions (goethite, hematite, pyrite), and silica nodules (chalcedony or opal). In some regions, especially desert or aeolian sandstones, iron oxide concretions known as Moqui marbles are well known.

Concretions occur most often in marine sedimentary rocks, including sandstones, shales, and limestones, but can also form in soils and diagenetically altered rocks. They can preserve fossils by forming around organic remains and may serve as stratigraphic markers. Weathering typically leaves the harder concretions as detached spheres or ovals.

Concretions are distinct from geodes, which are hollow and lined with crystals, and from nodules that blend more completely with the surrounding rock matrix.