NONTRONITE

Nontronite is a member of the smectite group of clay minerals characterized by its high iron(III) content, typically comprising over 30% Fe2O3 and less than 12% Al2O3 on an ignited basis. Unlike montmorillonite, nontronite has limited economic deposits. It shares with montmorillonite the ability to hold varying amounts of adsorbed water on its interlayer surfaces, along with exchangeable cations. Ignoring the isomorphic substitution of silicon by aluminum, the elemental composition of nontronite can be simplified to FeIII2Si4O10(OH)2.

A common structural formula for this mineral is Ca0.3(FeIII3.5Al0.4Mg0.1)(Si7Al0.8Fe0.2)O20(OH)4·nH2O.

Nontronite typically forms through the weathering of biotite and basalts, as well as from the precipitation of iron- and silicon-rich hydrothermal fluids, including those found in deep-sea hydrothermal vents. There is evidence that microorganisms may significantly influence its formation and are also involved in the reduction of structural iron in nontronite during anoxic soil conditions, with the reduced form of the clay showing high reactivity towards certain pollutants, potentially aiding in their environmental degradation. 

The mineral was named by Pierre Berthier in 1827, deriving its name from the town of Nontron in the Dordogne department of Nouvelle-Aquitaine, southwestern France, where it was first identified.