Notes on the Mineralogy and Origin of Glauconite

ABSTRACT The variability in the physical and chemical properties of pure glauconite seems to be less than is generally believed. The observed spread is partly attributed to insufficient purity of the material studied. One of the major causes of the variation in properties is the occurrence of non-structural iron oxides, which was established by electron microscopy and by chemical analysis of free iron oxide. Formulae for glauconite reported in the literature should be recalculated accordingly. Interpretations based on the relationship between iron content and geologic age appear doubtful. Very striking is the inverse relationship between the FeO- and Al2O3-contents of glauconite, which might indicate that part of the Fe occurs in the tetrahedral layers. Part of the Mg occupies vacant spaces in the octahedral layers. Titanium is the major trace element entering the glauconite structure. The common alteration product of glauconite is goethite, and under special conditions, jarosite. On heating glauconite to temperatures above 1000°C. maghemite is formed. The absence of glauconite in rocks older than late Precambrian is probably connected with the reducing character of the early atmosphere.