English: Magnesite - a chalky/earthy mass. (4.9 centimeters across at its widest)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5800 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The carbonate minerals all contain one or more carbonate (CO3-2) anions.

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.

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Photo gallery of magnesite:

www.mindat.org/gallery.php?min=2482