Magnesium is the eighth-most-abundant element in the Earth’s crust by mass and tied in seventh place with iron in molarity.[6] It is found in large deposits of magnesite, dolomite, and other minerals, and in mineral waters, where magnesium ion is soluble.
Although magnesium is found in more than 60 minerals, only dolomite, magnesite, brucite, carnallite, talc, and olivine are of commercial importance.
The Mg2+
cation is the second-most-abundant cation in seawater, which makes seawater and sea salt attractive commercial sources for Mg. To extract the magnesium, calcium hydroxide is added to seawater to form magnesium hydroxide precipitate.
MgCl
2 + Ca(OH)
2 Mg(OH)
2 + CaCl
2
Magnesium hydroxide (brucite) is insoluble in water and can be filtered out and reacted with hydrochloric acid to produced concentrated magnesium chloride.
Mg(OH)
2 + 2 HCl MgCl
2 + 2 H
2O
From magnesium chloride, electrolysis produces magnesium.
https://en.wikipedia.org/wiki/Magnesium#Occurrence
Intentionally altering atmospheric composition....what could go wrong?