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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
Environments:
Pegmatites
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).
Alteration
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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