Fluorine: its mineralogical residence in the oil shale of the Mahogany Zone of the Green River Formation, Piceance Creek Basin, Colorado, U.S.A.

Abstract

The Eocene oil shales of northwestern Colorado comprise one of the major resources of hydrocarbon fuels in the U.S.A. Oil shales of the Mahogany Zone (Parachute Creek Member, Green River Formation) contain up to 2100 mg/kg fluorine. The content of fluorine changes rapidly within short stratigraphic intervals, and, although not organically bound, correlates positively with the organic content of the rock. Studies of the mineralogical residence of fluorine in oil shale were made possible by combining the methods of low-temperature ashing, high-speed centrifugation in a liquid with a density gradient, X-ray diffraction, X-ray fluorescence, nuclear inelastic scattering, and electron microscopy. A major portion of the fluorine in the Mahogany Zone appears to be associated with the ubiquitous micaceous clay minerals, especially illite. Other fluorine-bearing minerals, such as fluorite (CaF 2), cryolite (Na 3AlF 6), and fluorapatite [Ca 5(PO 4) 3F] were not detected in the eleven samples studied. However, this does not exclude the possible presence of discrete crystals or crystal aggregates of these minerals in other portions of the Mahogany Zone.