Different ingredients and appearances.
1. Brucite is mainly composed of Magnesium hydroxide and magnesium oxide, and Magnesite is mainly composed of Magnesium carbonate.
2. Magnesite is cryptocrystalline with associated minerals such as dolomite and Opal. Brucite deposits occur in dolomite and magnesian limestone, and Brucite is disseminated or vein shaped.