What is Nontronite?
Nontronite is an iron-rich swelling smectite clay recognised by green colour and formation during low-temperature alteration of basalt and volcanic glass.
Mineralogy and crystal structure
Reference formula: Na0.3Fe2(Si,Al)4O10(OH)2·nH2O. Nontronite is a variable monoclinic smectite with interlayer water and exchangeable cations. The COD model is representative of a natural composition; hydration and iron-aluminium substitution vary substantially between deposits. Nontronite is classified with silicates and has a monoclinic crystal system. Typical hardness is 1–2 on the Mohs scale and specific gravity is approximately 2.0–2.6.
Colour, lustre and habit
Typical colours include green, yellow-green, brown, black. The usual earthy, massive, platy habit and earthy, dull, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Nontronite forms in or is associated with weathering of basalt, volcanic ash, submarine alteration and iron-rich hydrothermal systems. Important documented localities include France, the United States, Japan, New Zealand, Australia and South Africa.
Green clay from altered volcanic rock
Nontronite turns alteration into colour. Its green layers show how iron-rich fluids and water transform volcanic material into a swelling clay with environmental and geological significance.
Collecting and care
Keep dry and avoid dust; humidity changes can expand or soften the material.
