nontronite

Nontronite is an iron-rich clay mineral in the smectite group, a subset of the 2:1 phyllosilicates that form expandable clay minerals. It is typically described as a dioctahedral smectite in which iron, mainly Fe3+, substitutes for aluminum in the octahedral sheet, giving a characteristic iron-rich composition and color range from green to olive brown and red.

The mineral has the layered 2:1 structure common to smectites, consisting of two tetrahedral silica sheets sandwiching an octahedral sheet. In nontronite, the octahedral sheet is mostly occupied by iron, which influences its chemistry, cation exchange properties, and thermal stability. The interlayer space is hydrated and expandable, enabling swelling and high surface area.

Nontronite forms by weathering or alteration of iron-rich rocks and volcanic materials and is widespread in soils, sediments, and hydrothermal environments. It often occurs with other clays and iron oxides and is named after the town of Nontron in France. Its oxidation state and iron content influence its color and physical behavior, including its swelling in water and cation exchange capacity.

Analytically, nontronite is identified by X-ray diffraction as a 1:1 smectite pattern with a basal spacing near 1 nanometer that increases upon hydration. It exhibits typical smectite properties such as significant plasticity, swelling, and high surface area, but its iron content can affect stability and reactivity compared with aluminum-rich clays.

In soils and environmental contexts, nontronite is important as an indicator of redox conditions and as a participant in adsorption and immobilization processes for metals and contaminants.