No description of iddingsite has demonstrated that it has a definite chemical composition or optical properties. It has been identified largely on the basis of its association with remnants of olivine or as pseudomorphous replacement aggregates. A study of many rocks from New Mexico and some from Colorado containing iddingsite show varying degrees of alteration of olivine which is believed to be largely accomplished in the deuteric stage, and may or may not be continued in the weathering process. The alteration of olivine to iddingsite occurs largely in basaltic rocks whose modal proportion of olivine exceed their normative proportion of olivine because of fractional crystallization.
Study of iddingsite samples from New Mexico and Colorado by the x-ray powder diffraction method shows that goethite is the only crystalline phase and that the other substances shown by chemical analysis to occur in iddingsite are largely amorphous. Iddingsite in this case may therefore be regarded as a complex alteration product of olivine rather than a true mineral.