Natural Rust on Stone

Abstract

The iron content of the earth’s crust averages 5%. At the earth’s surface iron is tied up as green or black ferrous-ferric iron in the ferromagnesian silicates, as the black ferrous-ferric oxide magnetite, as the yellowish ferrous sulphides, pyrite and marcasite; as the grey to dark-brown ferrous carbonate siderite, and as the red or black ferric oxide hematite. The last is not only a common pigment in rock but can also accumulate as our most important iron ore. In humid atmospheres the brown to ochre-brown ferric hydroxide, goethite, is the most important mineral of the common “rust”, alpha-FeOOH. Goethite is usually accompanied by amorphous (not yet crystallized) ferric hydroxide of the same color. Natural rust is summarized as the “mineral” limonite which is not a mineral in the true sense. All the minerals mentioned here tend to adjust to the humid or semi-humid atmospheric surface conditions as they weather to ferric hydroxide or limonite. Metallic iron also changes to rust, mostly amorphous ferric hydroxide with some magnetic brown maghemite, gamma-Fe₂O₃. Crystallization or aging of the non-crystalline ferric hydroxide leads to the formation of submicroscopic goethite. In some rare instances deep orange gamma-FeOOH, lepidocrocite, can form.