Formula: (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2(OH)2.nH2O
Phyllosilicate (sheet silicate), smectite group, forms series with nontronite and with beidellite
Crystal System: onoclinic
Specific gravity: 2 to 3 measured
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
Solubility: Decomposed or gelatinised by common acids
Common impurities: Fe,K

Sedimentary environments
Hydrothermal 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 forms in a wide variety of sedimentary environments, and also in some granite pegmatites and in hydrothermal mineral deposits (Dana). It is a zeolite facies mineral.
Associated minerals include cristobalite, zeolites, biotite, quartz, orthoclase, dolomite, amphiboles, pyroxenes, olivine, calcite, gypsum, pyrite and limonite (HOM).


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 muscovite variety illite, aqueous SiO2 and H2O
Al2Si4O10(OH)2.nH2 + KAlSi3O8 → KAl2(AlSi3)O10(OH)2 + 4SiO2 + nH2O
(JVW p328)

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