The chemical composition of magnesite is MgCO3; this kind of crystal is a carbonate mineral belonging to the trigonal system. Usually magnesium is replaced by iron and manganese, but the iron content in natural magnesite is generally not high. Magnesite is usually grain-like or aphanitic. The latter type is also known as porcelain-like magnesite, which is white or gray, except for iron-based magnesite, which is yellow to brown, and has vitreous luster.