context: Oxygen is present in the atmosphere in trace quantities in the form of carbon dioxide (CO
  2). The Earth's crustal rock is composed in large part of oxides of silicon (silica SiO
  2, as found in granite and quartz), aluminium (aluminium oxide Al
  2O
  3, in bauxite and corundum), iron (iron(III) oxide Fe
  2O
  3, in hematite and rust), and calcium carbonate (in limestone). The rest of the Earth's crust is also made of oxygen compounds, in particular various complex silicates (in silicate minerals). The Earth's mantle, of much larger mass than the crust, is largely composed of silicates of magnesium and iron.
question: What is Fe2O?