Magnesite
Magnesite is a magnesium carbonate mineral that forms white to gray crystalline masses and, less commonly, rhombohedral crystals. It is an important source of magnesium and refractory magnesia.
Magnesite properties at a glance
| Type | Carbonates |
|---|---|
| Chemical formula | MgCO3 |
| Mohs hardness | 4 |
| Crystal system | trigonal |
| Luster | vitreous |
| Streak | White |
| Cleavage | Perfect rhombohedral |
| Fracture | Conchoidal to uneven |
| Specific gravity | 3 |
| Transparency | translucent |
| Common colors | White, Gray, Yellowish, Brown, Colorless |
| Rarity | common |
Geology & background
Magnesite forms by alteration of magnesium-rich rocks such as serpentinite, by metasomatism of carbonate rocks, and as a sedimentary precipitate in some lake and weathering settings.
Magnesite has been mined as the principal raw material for refractory bricks that line high-temperature furnaces, and it remains an industrial source of magnesium compounds.
How to identify Magnesite
- Look for white to gray crystalline or porcelain-like masses with perfect rhombohedral cleavage in crystalline material.
- Magnesite effervesces only in warm acid, which separates it from readily fizzing calcite.
- Distinguish it from dolomite by chemical testing and from calcite by its higher hardness and weaker acid reaction.
Where Magnesite is found
Magnesite occurs in altered magnesium-rich rocks and metasomatized carbonates worldwide, with notable sources in Austria, Brazil, China, Spain, and Australia.
How much is Magnesite worth?
| Entry-level specimen | $3 |
|---|---|
| Typical collector specimen | $25 |
| Premium specimen | $150+ |
Value depends on crystal form, color, size, locality, aesthetics, damage, and provenance.
Common uses
- Refractory magnesia
- Magnesium chemicals
- Dyed imitation gem material
- Industrial filler
- Collector specimens
Interesting facts
- Massive porcelain-like magnesite is often dyed to imitate turquoise in inexpensive jewelry.
- Magnesite reacts only with warm acid, unlike calcite, which fizzes readily in cold acid.
- Heating magnesite drives off carbon dioxide to produce magnesia, a key refractory material.
- Magnesite forms a partial chemical series with iron-bearing siderite.
Care & safety
Magnesite is moderately soft and can scratch or chip, and crystalline material cleaves easily.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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