NODULE (GEOLOGY)

A 'nodule' in petrology or mineralogy is an irregular rounded to spherical concretion. 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.
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 recrystalization of rhyolitic and obsidian flows. Thundereggs are geode like nodules that form in volcanic ash deposits during diagenesis.