Debates over iddingsite's non-definite crystal structure caused it to be de-listed as an official mineral by the IMA; thus, it is properly referred to as a rock.

It is a pseudomorph that has a composition that is constantly transforming from the original olivine, passing through many stages of structural and chemical change to create a fully altered iddingsite.

The geologic occurrence of iddingsite is limited to extrusive or subvolcanic rocks that are formed by injection of magma near the surface.

[1] Iddingsite is a pseudomorph, and during the alteration process the olivine crystals had their internal structure or chemical composition changed, although the external form has been preserved.

This is not true for all phases of the alteration of olivine because the atomic arrangement becomes distorted and causes a non-definite structure to form.

Iddingsite has a composition that is constantly transforming from the original olivine passing through many stages of structural and chemical change.

Iddingsite is an epimagmatic mineral derived during the final cooling of lava in which it occurs from a reaction between gases, water and olivine.

It is however dependent on oxidation conditions, hydration and the magma from which iddingsite forms must be rich in water vapor.

[5] The alteration of olivine to iddingsite occurs in a highly oxidizing environment under low pressure and at intermediate temperatures.

X-ray diffraction patterns found that there are five structural types of iddingsite that can occur during different stages of alteration.

Hematite has a triangular crystal system and experiences twinning by having an approximately hexagonal close-packed oxygen framework and has a structural orientation similar to olivine.

These alterations tend to be rare in iddingsite but when they are present they show a sharp diffraction spot making them easily identified.

[2] Iddingsite is a pseudomorph that usually has crystals rimmed by a thin zone of yellowish brown or greenish cryptocrystalline material.

An increase in beta refractive index, which typically is 1.9 can be seen in most types of iddingsite, as the alteration process proceeds.