A member of the calcite-type car-bonates having the formula MgCO₃. It forms dolomite (CaMg(CO₃)₂) with calcite (CaCO₃) in the system CaCO₃–MgCO₃. Pure magnesite is not common in nature because there exists a complete series of solid solutions between MgCO₃ and FeCO₃, which is constantly present in magnesite in its natural occurrence.