Redox and nonredox reactions of magnetite and hematite in rocks

Redox and nonredox reactions causing pseudomorphic replacement of hematite by magnetite and magnetite by hematite are compared.Pseudomorphic replacements resulting from redox reactions are known as martitization replacement of magnetite by hematite due to oxidation; reaction (1)] and mushketovitization replacement of hematite by magnetite due to reduction; reaction (2)]. These two replacements cause characteristic ore textures and volume changes (reaction (1): increase of 1.66%; reaction (2): decrease of 1.64%). These small volume changes are the reason that martitization and mushketovitization are widespread in many rocks under condition, however, that oxidizing or reducing fluids (solutions) are present.The same initial and end products may also be involved in nonredox reactions. Reaction (3) is the replacement of hematite by magnetite due to simple addition of Fe²⁺ atoms under basic conditions. This reaction causes an increase of the volume of 47.6%. Reaction (4), causing a volume decrease of 32.2%, is the replacement of magnetite by hematite due to leaching of Fe²⁺ atoms under acidic conditions. From these volume changes it is concluded that reaction (4) may occur in many rock types, whereas reaction (3) is restricted to unlithified sediments only. However, ore textures caused by nonredox reactions are unknown and therefore their occurrence in rocks is hypothetical.