A member of the smectite group of clay minerals, nontronite has the complicated structural formula of (CaO0.5,Na)0.3Fe3+2(Si,Al)4O10(OH)2·nH2O. Generally speaking, these minerals are around 30% Fe2O3 and less than 12% Al2O3. As a clay mineral, nontronite is also classified as a hydrous aluminum phyllosilicate and only forms in the presence of water.
Because the composition can vary, nontronites are often considered types of minerals, rather than a single mineral with a set composition.
Nontronite can range from green to yellow to brown in color, and rates a hardness of about 1.5-2 on the Mohs scale. It was named for its type locality at Nontron, Dordogne, France.
The only commercially viable nontronite mine is in New Zealand, where the finished products from the mine are used in applications such as paper and pulp, or clay cosmetics, though the mineral can be found in many other locations.