Nodules and Concretions Simply Explained
In mineralogy a nodule is a small rounded lump of one single mineral or possibly a combination of minerals which 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 slowly 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 into the glass.
Environmental changes can cause minerals to precipitate. In the desert water evaporates leaving salts to precipitate on dried out bodies of water whilst below the earth's surface 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 rocks. The most common mineral cements include silica, calcite, limonite, hematite and clay minerals.
Precipitated salt on a dried lake bed after the water evaporated
The most important difference between a concretion and a nodule is that concretions are formed from mineral precipitation around a formaton 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.