Iron Oxides in Petroferric Materials

Abstract Samples of petroferric materials with a very wide range of chemical and morphological characteristics were obtained from Australia, Bolivia, Brazil, Nigeria, Thailand, and the USA. A prominent feature of all samples is a high content of Fe and/or Al oxides, relative to other constituents. Free Fe oxide is present mainly as goethite or hematite with very small amounts of lepidocrocite in some samples. Soil sample color is related to the Fe oxide mineralogy; the reddish colored samples have a high content of hematite while the yellowish samples have a high goethite content. Also, samples with a low active Fe ratio tend to have a reddish color while those with a high active Fe ratio are yellowish in color. The active Fe ratio is not predictable from a knowledge of the crystallinity or particle size of goethite present, but the presence of lepidocrocite generally increases the oxalate‐extractable Fe. Both hematite and goethite in the samples contain Al up to about 25 atom percent. Treatment of the samples with boiling 5 M NaOH decreased the Al‐content of some but not all hematites and goethites.