Magnesite is a magnesium carbonate mineral (MgCO3). It forms a chemical spectrum with dolomite (CaMg(CO3)2) and calcite (CaCO3). It often forms coarsely-crystalline masses or chalky/earthy masses. It has a nonmetallic luster and can be any color, but is often whitish. It has a white streak, a hardness of about 3.5 to 4.5, and rhombohedral cleavage (= same cleavage as calcite).
Magnesite is usually the result of metamorphic chemical alteration of Mg-bearing rocks such as peridotites and serpentinites by the action of carbonic acid-rich fluids.
Magnesite vein (white) in serpentinite (dark green) (4.7 cm across). The magnesite here is derived from alteration of the host serpentinite rock by CO3-2-bearing fluids. The serpentinite is metamorphosed oceanic lithospheric dunite hosted in schists and quartzites of the Ottauquechee Formation (Lower Cambrian). Metamorphism of the original mantle dunite rock occurred during the Taconic Orogeny and Acadian Orogeny (Early & Middle Paleozoic).
Locality: J.A. Vermont Verde Antique International Quarry, eastern side of Quarry Hill Road, northeast of Rochester, northwestern Windsor County, central Vermont, USA (43° 54' 55" North, 72° 48' 26" West).
Magnesite (9.9 cm across) from the Grenville Front area of western Quebec, Canada. This monocrystalline magnesite rock looks like marble, but it's not composed of calcite (it won't bubble in cold acid). Magnesite will never form with calcite. A monocrystalline magnesite rock is called magnesitite.
Magnesite (4.9 cm across) - a chalky/earthy mass.