Encyclopedia of Chemistry

A nodule in petrology or mineralogy is a secondary structure, generally spherical or irregularly rounded in shape. Nodules are typically solid replacement bodies of chert or iron oxides formed during diagenesis of a sedimentary rock. Nodules may be hollow as geodes or vugs or filled with crystals and intricate geometric shrinkage patterns as in septarian nodules.

There is an important distinction to draw between nodules and concretions; the latter being formed from mineral precipitation around some kind of nucleus while the former is a replacement body.

Manganese nodules are rounded accumulations of manganese, iron and other metal oxides that accumulate on the deep ocean floor.

Silicic volcanic rocks also form nodular segregations during cooling and recrystallization of rhyolitic and obsidian flows. Thundereggs are geode like nodules that form in volcanic ash deposits during diagenesis.