What are Nodules and Concretions?

Nodules and Concretions - Simply Explained

In mineralogy, a nodule is a rounded mass of one single mineral or a combination of minerals that occurs within a host rock. These are generally harder than the rock or sediment in which they're found.  

A concretion is a rounded mass of compacted material that forms when minerals precipitate around a central nucleus within sediment.

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Precipitation is a process whereby dissolved minerals are separated from the water in which they're being carried. This happens when water evaporates or recedes.

Imagine a glass of water containing salt that's fully dissolved being placed somewhere very hot. As the water evaporates, the concentration of salt increases to the point where it's no longer being dissolved by the water. As the water dries up completely, the salt precipitates in the glass.

When a body of water can no longer contain dissolved minerals and salts, the particles come together and form a solid crystal.

Environmental changes can cause minerals to precipitate. In the desert, water evaporates, leaving salts to precipitate on dried-out bodies of water.

Below Earth's surface, dissolved minerals being carried in groundwater precipitate to form mineral cement as the water dries up. The process of cementation hardens the sediments which over millions of years go on to form sedimentary rock.

The most common mineral cements include silica, calcite, limonite, hematite, and clay minerals.

The most important difference between a concretion and a nodule is that concretions form through mineral precipitation around a central nucleus, whereas a nodule is a self-contained mass within a host rock that has a different composition.

Article Picture

The picture at the top of our article is azurite nodules. Courtesy of Stan Celestian.

The second photo is precipitated salt on a dried lake bed in Death Valley after the water has evaporated.