Biotite-Glauconite Transformation and Associated Minerals

Abstract In examining a number of glauconitic sediments, a surprising amount of evidence is found tying glauconite to a biotite or iron-mica derivation. Principal evidence for relationship lies in series of transition grains demonstrating gradual change from mica to glauconite. A most interesting group has been illustrated before (3, p. 1584), but is shown here again since it constitutes one of the best examples from recent sediments (Fig. 1). Groups similar to it have been found in glauconitic sediments collected from about one hundred localities spread over a dozen states and countries. They are not limited to recent material alone, but range down the column as low as Cambrian. Several illustrations of such fossil biotite-glauconite associations are in preparation. In addition to individual grains which show the transformation proccess, another line of evidence for the biotite-glauconite relationship is the change in facies in some shelf sediments. By change in facies is meant the gradual change from biotite-rich sands fairly near shore to mixed glauconite-mica silty sands farther off shore and a little deeper, thence progressively to glauconite muds at a depth of about 100-fathoms. A map and description of this type of occurrence in Monterey Bay have been given (2, p. 1354; 3, p. 1570). The most potent evidence that indicates intimate relationship of the minerals however, is Gruner’s X-ray work (5). Gruner, in addition to presenting experimental data, gives an interesting discussion and comparison of ionic arrangement and structure of unit cells and concludes that glauconite is a mica in structure (p. 699).