IPA: /mæɡˈniːʒən/
KK: /mægˈniːʒən/
Relating to or containing magnesium, especially in minerals.
The geologist studied the magnesian rocks to understand their composition.
Magnesian is derived from "magnesia" (from Latin *magnesia*, meaning a type of mineral) and the suffix "-an" (meaning pertaining to). The term refers to something that is related to or derived from magnesia, a mineral containing magnesium.
Think of 'magnesia' as the mineral that gives its name to 'magnesian', which means something related to that mineral.