Mineral Guide · Mineral

Glaucophane

Glaucophane is a blue sodium-magnesium amphibole and a classic indicator of high-pressure, low-temperature metamorphism.

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Crystal structure

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Mineral properties

Formula: Na₂Mg₃Al₂Si₈O₂₂(OH)₂
Class: Silicates
Crystal system: Monoclinic
Typical colours: grey, blue, blue-grey, lavender
Lustre: Pearly, Vitreous
Hardness: 6.00-6.50
Specific gravity: 3.00-3.20
Crystal habit: Fibrous, Granular, Prismatic
Origin: blueschists, subduction-zone metamorphic rocks and sodium-rich metamorphic assemblages

What is Glaucophane?

Glaucophane is a blue sodium-magnesium amphibole and a classic indicator of high-pressure, low-temperature metamorphism.

Mineralogy and structure

Reference formula: Na₂Mg₃Al₂Si₈O₂₂(OH)₂. Glaucophane has the monoclinic amphibole structure. Its blue colour and association with blueschist facies are useful geological clues, but composition can vary. Glaucophane belongs to the silicates and has a monoclinic crystal system. Typical hardness is 6–6.5 on the Mohs scale and specific gravity is approximately 3.0–3.2.

Colour and appearance

Typical colours include blue, blue-grey, lavender, grey. The usual prismatic, fibrous, granular habit and vitreous, pearly lustre are useful descriptive features, but colour alone is not diagnostic.

Formation and localities

Glaucophane forms in or is associated with blueschists, subduction-zone metamorphic rocks and sodium-rich metamorphic assemblages. Important documented localities include Japan, California, Greece, Italy, New Caledonia and Turkey.

Collecting and care

Protect fibrous or cleavable aggregates from impact and keep the metamorphic locality with the specimen.

Sources and editorial review

Reviewed by NotOnlyRocks Editorial Team

Last reviewed: 31 August 2026

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