Formula: Na0.3Fe3+2(Si,Al)4O10(OH)2.nH2O
Phyllosilicate (sheet silicate), smectite group
Specific Gravity: 2.06 to 2.32
Hardness: 1 to 2
Streak: White
Colour: Green, olive-green, yellow-green, yellow, orange, brown
Common impurities: Ti,Mg,Ca

Volcanic igneous environments
Metamorphic environments
Hydrothermal environments

Nontronite is a weathering product of basalt, kimberlite and other mafic and ultra-mafic volcanic rocks. It also occurs in poorly drained volcanic ash soils, in some hydrothermally altered mineral deposits, mid ocean ridge basalt and contact metamorphosed limestone. It is authigenic (formed in place) in recent marine sediments. It is formed in the presence of both neutral and acid cool hydrothermal fluids, and is stable up to about 140oC. Associated minerals include quartz, opal, hornblende, pyroxene, olivine, mica and kaolinite (AofA, HOM).


The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. nontronite is a secondary silicate that occurs as an alteration product of andradite (AJM 22.1.38).

The type locality is Le Manderau, Saint-Pardoux-la-Rivière, Nontron, Dordogne, Nouvelle-Aquitaine, France.

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, nontronite occurs as a massive alteration product of spodumene. The Emmons pegmatite is an example of a highly evolved boron-lithium-cesium-tantalum enriched pegmatite (R&M 94.6.512).

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