Magnesite is magnesium carbonate, MgCO3. Iron (as Fe2+) substitutes for Mg with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts. Dolomite, (Mg,Ca)CO3, is almost indistinguishable from magnesite.
Magnesite occurrs as veins in and an alteration product of serpentine and other magnesium rich rock types in both contact and regional metamorphic terranes. These magnesites often are cryptocrystalline and contain silica as opal or chert.
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