Stages of Glauconite Formation in Modern Foraminiferal Sediments

Abstract

ABSTRACT Examination of 325 bottom samples taken at 68 stations off the southeast coast of the United States by the United States Fish and Wildlife Service revealed the common occurrence of glauconite in foraminiferal sands. The glauconite is variable in color and in morphology, ranging from dark greenish-black to greenish-white. The dark green glauconite generally occurs in the smooth, bulbous pellet form, whereas the lighter green glauconite commonly occurs as rough, porous pellets in the form of internal molds of microorganisms. However, in some internal mold pellets, dark green and light green glauconite occur together. Magnetic fractions were taken from all samples. The samples composed primarily of foraminiferal tests yielded a glauconite-rich sparate. In the magnetic separates occurred many intact shells of foraminifers which, upon solution in acid, generally yielded internal molds of the light-colored glauconite. Analysis by x-ray diffraction of single pellets and of composite samples of hand-picked grains revealed the glauconite to be essentially a mica-type clay varying in degree of hydration and disorder. Examination of oriented slides of the composite samples following vapor glycol treatment showed that the dark-green glauconite was unaffected, but that the pale green glauconite was expandable. Debye-Scherrer films of single pellets of the light green glauconite showed the presence of kaolinite which was not found in the darker green glauconite. A decrease in sharpness and a shift of the basal mica diffraction line to higher d values as well as a decrease in the potassium content and an increase in the water content were directly related to the color change of the glauconite from dark green to pale green. The glauconite in these foraminiferal sands is interpreted as representing various stages of glauconitization of clay material which was originally enclosed within the tests of microorganisms.