Mineral Guide · Mineral

Hercynite

Hercynite is the iron-aluminium spinel, a hard dark oxide associated with high-temperature metamorphic rocks and aluminium-rich deposits.

Interactive 3D structure

Crystal structure

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Mineral properties

Formula: FeAl₂O₄
Class: Oxides
Crystal system: Cubic
Typical colours: black, green-black, brown-black
Lustre: Sub-metallic, Vitreous
Hardness: 7.50-8.00
Specific gravity: 4.30-4.50
Crystal habit: Granular, Massive, Octahedral
Origin: contact-metamorphic rocks, granulites, corundum-bearing assemblages and aluminium-rich sediments

What is Hercynite?

Hercynite is the iron-aluminium spinel, a hard dark oxide associated with high-temperature metamorphic rocks and aluminium-rich deposits.

Mineralogy and structure

Reference formula: FeAl₂O₄. Hercynite has the cubic spinel structure and commonly contains magnesium, chromium or zinc substitutions. It is often granular, so the host rock is part of its identification. Hercynite belongs to the oxides and has a cubic crystal system. Typical hardness is 7.5–8 on the Mohs scale and specific gravity is approximately 4.3–4.5.

Colour and appearance

Typical colours include black, green-black, brown-black. The usual octahedral, granular, massive habit and vitreous, sub-metallic lustre are useful descriptive features, but colour alone is not diagnostic.

Formation and localities

Hercynite forms in or is associated with contact-metamorphic rocks, granulites, corundum-bearing assemblages and aluminium-rich sediments. Important documented localities include Austria, Germany, India, South Africa, Sri Lanka and the United States.

Collecting and care

Hard but brittle at edges; protect octahedral faces and avoid assuming a spinel species from colour alone.

Sources and editorial review

Reviewed by NotOnlyRocks Editorial Team

Last reviewed: 31 August 2026

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