Glaucophane

glaucophane

jadeite

montmorillonite

lawsonite

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Formula: ☐Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂
Inosilicate (chain silicate) amphibole supergroup
Crystal System: Monoclinic
Specific gravity: 3.08 to 3.22 measured, 3.123 calculated
Hardness: 5½ - 6½
Streak: Light blue
Colour: Blue-grey, blue-lavender, blue-black
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Common impurities: Li,Ti,Cr,Mn,Ca,K,F,Cl
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) + (Mg_4.0Fe_2.0)(AlSi₃O₁₀)(OH)₈ + CaCO₃ → 5(Na_0.8Ca_0.2)(Mg_0.2Al_0.8Si₂)₆ + 2Na₂(Mg₃Al₂)(Al_0.5Si_7.5)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) + 2Ca_0.5(Mg_3.5Al_0.5)Si₈O₂₀(OH)₄.nH₂O → 5(Na_0.8Ca_0.2)(Mg_0.2Al_0.8Si₂)₆ + 2Na₂(Mg₃Al₂)(Al_0.5Si_7.5)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).

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