Glaucophane

inosilicate

metamorphic

Formula: ☐Na2(Mg3Al2)Si8O22 (OH)2 inosilicate (chain silicate) amphibole
Specific gravity: 3.0 to 3.3
Hardness: 5½ - 6½
Streak: Light blue
Colour: Blue-grey, blue-lavender, blue-black
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Metamorphic environments

Glaucophane is found only in metamorphic rocks such as schist, eclogite and marble. It is a low-temperature, relatively high pressure metamorphic mineral, occurring in association with jadeite, lawsonite and aragonite. It is a major constituent of glaucophane schist, and it is a characteristic mineral of the blueschist facies.

Alteration

albite, chlorite and calcite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz, CO2 and H2O
8Na(AlSi3O8) + (Mg4.0Fe2.0)(AlSi3O10)(OH)8 + CaCO3 → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 2SiO2 + CO2 + 2H2O
In low to intermediate metamorphism jadeite-glaucophane assemblages may arise from reactions such as the one above (DHZ 2A p475).

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).

antigorite and albite to glaucophane and H2O
Mg3Si2O5(OH)4 + 2Na(AlSi3O8) → ☐Na2(Mg3Al2)Si8O22(OH)2 + H2O
This is a metamorphic reaction (DHZ 3 p156).

chlorite (clinichlore), actinolite and albite to glaucophane, iron-poor epidote, SiO2 and H2O
9Mg5Al(AlSi3O10)(OH)8 + 6☐Ca2Mg5Si8O22(OH)2 + 50Na(AlSi3O8) → 25☐Na2(Mg3Al2)Si8O22(OH)2 + 6Ca2Al3[Si2O7][SiO4]O(OH) + 7SiO2 + 14H2O
This is a metamorphic reaction (DHZ 3 p156).

Common impurities: Li,Ti,Cr,Mn,Ca,K,F,Cl

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