Mineral Guide · Mineral

Hydromagnesite

Hydromagnesite is a hydrated magnesium carbonate that forms white fibrous crusts, rosettes and earthy masses in magnesium-rich environments.

Interactive 3D structure

Crystal structure

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Mineral properties

Formula: Mg₅(CO₃)₄(OH)₂·4H₂O
Class: Carbonates
Crystal system: Monoclinic
Typical colours: colourless, white, grey, yellowish
Lustre: Dull, Pearly, Silky
Hardness: 3.50-4.00
Specific gravity: 2.20-2.30
Crystal habit: Botryoidal, Earthy, Fibrous
Origin: serpentinite weathering zones, caves, alkaline soils and magnesium-rich springs

What is Hydromagnesite?

Hydromagnesite is a hydrated magnesium carbonate that forms white fibrous crusts, rosettes and earthy masses in magnesium-rich environments.

Mineralogy and structure

Reference formula: Mg₅(CO₃)₄(OH)₂·₄H₂O. Hydromagnesite contains both hydroxyl groups and water in its carbonate structure. It can dehydrate or alter with heat, so the specimen should be treated as a delicate hydrated carbonate. Hydromagnesite belongs to the carbonates and has a monoclinic crystal system. Typical hardness is 3.5–4 on the Mohs scale and specific gravity is approximately 2.2–2.3.

Colour and appearance

Typical colours include white, colourless, grey, yellowish. The usual fibrous, botryoidal, earthy habit and pearly, silky, dull lustre are useful descriptive features, but colour alone is not diagnostic.

Formation and localities

Hydromagnesite forms in or is associated with serpentinite weathering zones, caves, alkaline soils and magnesium-rich springs. Important documented localities include Austria, Greece, Italy, the United States, Canada and South Africa.

Collecting and care

Keep dry and away from heat; protect fibrous surfaces from abrasion and do not clean with acids.

Sources and editorial review

Reviewed by NotOnlyRocks Editorial Team

Last reviewed: 31 August 2026

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