What are Nodules and Concretions?
Nodules and Concretions Simply Explained
In mineralogy a nodule is a small rounded lump of one single mineral or a combination of minerals. These are generally harder than the rock or sediment in which they're found.
A concretion is a collection of hardened compacted matter that forms as a result of mineral precipitation in spaces between the particles.
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.
Crystals form from the precipitation from water. When a body of water can no longer contain dissolved minerals and salts the particles come together and form a solid crystal.
This process is known as precipitation.
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 concretions are formed from mineral precipitation around a formation of matter (a substance that has mass and takes up space). A nodule is a self-contained mass or lump that exists within a host rock but has a different composition.
Article Picture
The picture at the top of our article is azurite nodules. The photo is clickable and redirects to the original image. Courtesy of Stan Celestian.
The second photo is precipitated salt on a dried lake bed in Death Valley after the water has evaporated.
