Mineral Guide · Mineral

Grandidierite

Grandidierite is a rare magnesium-aluminium borosilicate, prized for blue-green pleochroism and transparent material from a few localities.

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Crystal structure

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Mineral properties

Formula: (Mg,Fe)Al₃BSiO₉
Class: Silicates
Crystal system: Orthorhombic
Typical colours: green, blue-green, grey, blue
Lustre: Vitreous
Hardness: 7.00-7.50
Specific gravity: 2.80-3.00
Crystal habit: Granular, Massive, Prismatic
Origin: boron-rich metamorphic rocks, granulites and aluminium-rich high-grade assemblages

What is Grandidierite?

Grandidierite is a rare magnesium-aluminium borosilicate, prized for blue-green pleochroism and transparent material from a few localities.

Mineralogy and structure

Reference formula: (Mg,Fe)Al₃BSiO₉. Grandidierite is orthorhombic and compositionally related to ominelite. Strong pleochroism and documented locality are valuable identification clues. Grandidierite belongs to the silicates and has a orthorhombic crystal system. Typical hardness is 7–7.5 on the Mohs scale and specific gravity is approximately 2.8–3.0.

Colour and appearance

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

Formation and localities

Grandidierite forms in or is associated with boron-rich metamorphic rocks, granulites and aluminium-rich high-grade assemblages. Important documented localities include Madagascar, Sri Lanka, Malawi, Namibia and limited other occurrences.

Collecting and care

Good hardness but protect transparent edges and preserve provenance for rare gem material.

Sources and editorial review

Reviewed by NotOnlyRocks Editorial Team

Last reviewed: 31 August 2026

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