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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.
Additional recommended knowledge
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.
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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nodule_(geology)". A list of authors is available in Wikipedia.|