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Magnesite is a magnesium carbonate which belongs to the group of calcite. It forms a series with siderite, with "mesitite" being "half-way" in composition.
Magnesite is mainly due to the result of three geological processes: weathering of rocks rich in magnesium by carbonated water; then limestone metamorphism and finally metasomatism.
Transparent to translucent, with a vitreous luster, magnesite has a perfect cleavage according to {10-11}. It has a colourless to white color but can also be tinted in yellow to brown. Magnesite forms compact masses with granular, rhombohedra with the {10-11} face being very developed and lenticular crystals.
The good crystals of this frequent mineral are rare: the best were found in Brumado (Bahia, Brazil) as gemmy rhombohedra, slightly yellowish (up to 30 cm along their edge). Magnesite was also found in large white-grayish crystals (partially gemmeous) in Oberdorf an der Laming, Styria and as brown centimetric rhombohedra, included in the Pfitschtal schists, the Tyrol, Austria. Other superb crystals reaching 15mm were found at the Val Malenco, Italy.
In France, magnesite is known in the district of Brioude-Massiac (Azérat, Lavoûte-Chilhac, Saint-Beauzire, Lubilhac...), with Champagnat-le-Jeune (Puy-de-Dôme), in Monviso (Briançon, Hautes-Alpes), at la Mure (Isère), Peisey-Nancroix (Savoie) has and has Ponts-Tarrets (Rhône).
The species is efined in 1808 out of two deposits: Magnisía (Magnesia) in Thessalia (Greece) and Bettolino, Baldissero Canavese (Canavese, area of Turin, Piedmont, Italy). The localization of this type is not known.
HISTORY : Name inspired from the element magnesium (Mg) and its type-locality: Magnesia in Greece
Species first described in 1808 by Dietrich Ludwig Gustav Karsten (1768-1810), German mathematician
Type-locality: Magnesie, Thessaly, Greece
CHEMICAL FORMULA : Mg CO3
CRYSTAL SYSTEM : Tetragonal
COLOR : Colourless, white, white-grayish, brown, pink
DIAPHANIETY : Transparent to translucent
LUSTER : Vitreous
STREAK : White
MORPHOLOGIE : Rhomboedric crystals or scalénohedras. As compact masses
HARDNESS : 3,5-4,5
CHEMICAL CLASS: 3,01
DENSITY : V - Carbonates, nitrates and borates
GROUP : Calcite
STRUNZ CLASS BEFORE 2001 : 5/B.02-30
STRUNZ CLASS AFTER 2001 : 5.AB.05
