Eckermannite

eckermannite

jadeite

albite

glaucophane

Images

Formula: NaNa₂(Mg₄Al)Si₈O₂₂(OH)₂
Inosilicate (chain silicate), eckermannite root name group, sodium amphibole subgroup, amphibole supergroup,
Crystal System: Monoclinic
Specific gravity: 3.02 calculated
Hardness: 5 to 6
Streak: Greenish white
Colour: Blue-green
Environments

Metamorphic environments

Localities

At the type locality, the Hpakant-Tawmaw Jade Tract, Hpakant Township, Mohnyin District, Kachin State, Myanmar, eckermannite occurs in a jadeite-amphibole rock, likely formed under blueschist facies conditions. Associated minerals include jadeite and albite (HOM).

Alteration

The implications are that the stability of sodium amphibole at very high pressures (>4 GPa) involves eckermannitic rather than glaucophanic compositions. This highly sodium-rich amphibole can occur even in bulk compositions that are not particularly sodium-rich. In blueschist facies metamorphic rocks, it is possible that eckermannite forms by the reaction:

9 jadeite + 7 talc → 3 eckermannite + 3 pyrope + 13 coesite + 4 H₂O
9NaAlSi₂O₆ + 7Mg₃Si₄O₁₀(OH)₂ → 3NaNa₂(Mg₄Al)Si₈O₂₂(OH)₂ + 3Mg₃Al₂(SiO₄)₃ + 13SiO₂ + 4H₂O

above the stability limit of glaucophane that is defined by the reaction:

glaucophane → 2 jadeite + talc
☐Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂ → 2NaAlSi₂O₆ + Mg₃Si₄O₁₀(OH)₂

(AM 103.989–992).

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