Carbonyl Iron Caplets - Carbonyl iron is pure iron micro particles. It is 100% elemental iron and may be gentler on the system then iron salts.* Each Carbonyl Iron Caplet contains 45mg of elemental iron. Ferrous Sulfate Tablets - Ferrous Sulfate is an iron salt. It is 20% elemental iron and is the type of iron most familiar to doctors, making it the most commonly recommended. Each Ferrous Sulfate Tablet contains 65mg of elemental iron.