clay mineral

Formula: (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2(OH)2.nH2O
Phyllosilicate (sheet silicate)
Specific gravity: 2.0 to 2.7
Hardness: 1 to 2
Streak: White
Colour: White, buff, yellow, green, rarely pale pink to red. Pink to red coloration is due to high valance Mn

Sedimentary environments

Montmorillonite is a common clay mineral. It is an alteration product of volcanic tuff and ash, or it may precipitate from water. It forms under alkaline conditions of poor drainage, and is stable up to about 140oC (AofA). It is a zeolite facies mineral.


albite and montmorillonite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz and H2O
8Na(AlSi3O8) + 2Ca0.5(Mg3.5Al0.5)Si8O20(OH)4.nH2O → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 15SiO2 + 6H2O
This reaction occurs in low to intermediate metatmorphism (DHZ 2A p 475)

montmorillonite and K-feldspar to illite, aqueous SiO2 and H2O
Al2Si4O10(OH)2.nH2 + KAlSi3O8 → KAl2(AlSi3)O10(OH)2 + 4SiO2 + nH2O
(JVW p328)

Common impurities: Fe,K

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