Class : Carbonates, nitrates, borates
    Subclass : Anhydrous carbonates
    Crystal system : Trigonal
    Chemistry : MgCO₃
    Rarity : Common

Magnesite (or giobertite) is a carbonate of the calcite group. It forms a continuous series with siderite but not with calcite, since its CaO content does not exceed 6%. The intermediate terms of the magnesite - siderite series were formerly referred to as breunnerite and mesitine. Manganese, cobalt and nickel can also be present at low levels. Magnesite can result from different geological processes : alteration of magnesium-rich rocks (mainly serpentinites) by carbonate waters ; magnesium metasomatosis of limestones with passage through an intermediate stage of dolomitization ; more rarely evaporites. It is exceptional as a matrix for hydrothermal veins. It was named thus in reference to its chemical composition, the term magnesium itself comes from the Greek magnesia lithos (stone of Magnesia), Magnesia being a region of Thessaly in Greece. Transparent to translucent and usually vitreous in luster, magnesite has perfect cleavage according to {10-11}. It is colorless, white to bluish-white, sometimes yellow to brown, and forms compact to granular masses, or rhombohedra, more rarely hexagonal prisms. It is a common magnesium ore that was formerly used in pyrotechnics and photography. By calcination it produces magnesia (MgO). Magnesia is mainly used in the refractory industry, particularly for the interior linings of steel furnaces, and also in the chemical industry. Magnesite has additional uses in porcelain, ceramics, rubber, stationery and quick-setting cements. Finally, it should be noted that it is massive and is cut and polished into cabochons and trinkets and sold under the trade name "howlite" and "pinolite" (or chrysanthemum stone).

Main photo : Magnesite from Brumado, Bahia, Brazil © Rob Lavinsky

Magnesite can present twins on {01-12}.

Massive magnesite sold under the name "howlite" is often dyed in various colors, notably blue to imitate turquoise.