Magnesite is a carbonate mineral, magnesium carbonate (MgCO3). It is an alteration product of calcite or dolomite in sedimentary rocks and is found in microcrystalline masses and hexagonal crystals like Iceland spar. Magnesite is white, with a glassy to dull texture. Hardness 3.5–5, relative density 3.1. Magnesite is used mainly for lining furnaces, since it decomposes to give magnesium oxide on heating.