You can get Magnesium Oxide or Magnesium Hydroxide which by the way is what the expensive "DOANS" pills are made of!
Or if you can get Magnesium Carbonate, it is just a form of Lime which is low in Calcium Carbonate!
And the Carbonate has Oxygen in it's compound!
When the average person does not understand any Basic Chemistry, you may give all of your Hard earned Cash away for Fancy names etc, when "IF" you understood some Basic Chemistry you would not be Hood Winked so easily!