Magnesite is a mineral with the chemical formula MgCO₃ (magnesium carbonate). Mixed crystals of iron(II) carbonate and magnesite (mixed crystals known as ankerite) possess a layered structure: monolayers of carbonate groups alternate with magnesium monolayers as well as iron(II) carbonate monolayers.   Manganese, cobalt and nickel may also occur in small amounts.