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Formula: Na0.3Fe3+2(Si,Al)4O10(OH)2.nH2O
Phyllosilicate (sheet silicate), smectite group
Crystal System: Monoclinic
Specific Gravity: 2.2 to 2.3 measured
Hardness: 1 to 2
Streak: White
Colour: Green, olive-green, yellow-green, yellow, orange, brown
Common impurities: Ti,Mg,Ca
Environments:
Volcanic igneous environments
Pegmatites
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).
Localities
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).
At the Santa Rosa pegmatite district, Itambacuri, Minas Gerais, Brazil, nontronite occurs filling cavities in
pegmatites, often in association with
tourmaline, in the Santa Rosa mine
(MinRec 56.4.451).
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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