Definitions for iddingsite
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A mixture of clay minerals and iron oxides formed by alteration of olivine
Iddingsite is an alteration of olivine that consists of a mixture of clay minerals, iron oxides and ferrihydrites. It is absent from deep-seated rocks and is found on meteorites. As it has been found on Martian meteorites, its ages have been calculated to obtain absolute ages when liquid water was at or near the surface of Mars. Iddingsite forms from the weathering of basalt in the presence of liquid water and can be described as a phenocryst, i.e. it has megascopically visible crystals in a fine-grained groundmass of a porphyritic rock. It is a pseudomorph that has a composition that is constantly transforming from the original olivine that pass though many stages of structural and chemical change to create a fully altered iddingsite. Because iddingsite is constantly transforming it does not have a definite structure or a definite chemical composition. The chemical formula for iddingsite has been approximated as MgO * Fe2O3 * 4 H2O where CaO can be substituted by MgO. The geologic occurrence of iddingsite is limited to extrusive or subvolcanic rocks that are formed by injection of magma near the surface.
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