This chapter contains sections titled: Introduction Sedimentary Magnesite—Basis for Carbonate Deposition Serpentine Alteration by Hydrothermal Processes Cryptocrystalline Magnesite Formation by Infiltration Crystalline Magnesite—Replacement of Limestone and Dolomite Brucite Worldwide Occurrence of Magnesite and Brucite Physical and Chemical Properties of Magnesite Chemical and Physical Properties of Brucite Bibliography References