It's mostly in the form of ferrous oxide (Fe(+2)). Fully oxidized to rust, it goes into the ferric form (Fe(+3)). It's very common in weathered rock to see th

It's mostly in the form of ferrous oxide (Fe(+2)). Fully oxidized to rust, it goes into the ferric form (Fe(+3)). It's very common in weathered rock to see the red color of ferric iron. Ferrous iron occurs in common minerals like pyroxene, hornblende, pyrite, and olivine. Olivine is the most common mineral in the upper mantle.