Magnesium is not found in it's metallic state because it reacts readily with oxygen, dilute acids and hot water. Magnesium ions make up 0.13% of sea water and is extracted using the Dow process.
There are over 60 minerals in which magnesium is found. Brucite, carnallite, dolomite, magnesite, olivine and talc are the important commercial ones. Note that magnesium can also be produced from...