carnallite ore is the raw material from which magnesium is obtained electrolytically.
British Dictionary definitions for carnallite
a white or sometimes coloured mineral consisting of a hydrated chloride of potassium and magnesium in orthorhombic crystalline form: a source of potassium and also used as a fertilizer. Formula: KCl.MgCl2.6H2O
C19: named after Rudolf von Carnall (1804–74), German mining engineer; see -ite1