Iron comes in 2 forms: heme iron (from meat and animal products) and non-heme iron (from other sources like vegetables and iron supplements). Non-heme iron used in iron supplements can appear in 3 forms:
1 Gordeuk VR, Brittenham GM, Hughes M, Keating LJ, Opplt JJ. High-dose carbonyl iron for iron deficiency anemia: a randomized double-blind trial. Am J Clin Nutr. 1987;46(6):1029-1034.
2 Gordeuk VR, Brittenham GM, McLaren CE, Hughes MA, Keating LJ. Carbonyl iron therapy for iron deficiency anemia. Blood. 1986;67(3):745-752.