MAGNESITE
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.