1. A PROCESS SUITABLE FOR PRODUCING CRYSTALLINE MAGNESIUM FLUORIDE COMPRISING: (A) FORMING A REACTION MIXTURE CONSISTING ESSENTIALLY OF AMMONIUM BIFLUORIDE, AN AQUEOUS SOLUTION OF AMMONIUM HYDROXIDE AND MAGNESIUM CARBONATE, SAID AMMONIUM BIFLUORIDE AND AMMONIUM HYDROXIDE BEING IN STOICHIOMETRIC EXCESS OF SAID MAGNESIUM CARBONATE; (B) HEATING THE FOREGOING MIXTURE AT A TEMPERATURE OF AT LEAST ABOUT 60*C. FOR A TIME SUFFICIENT TO FORM AN INTERMEDIATE SOLID COMPLEX; 8C) SEPARATING SAID COMPLEX FROM THE AQUEOUS PHASE; (D) DRYING SAID COMPLEX AT A TEMPERATURE OF AT LEAST ABOUT 100*C. AND (E) HEATING THE DRIED COMPLEX TO AT LEAST ABOUT 350* C. TO FORM MAGNESIUM FLUORIDE.