Trace and minor elements in Green River oil shale (Colorado, U.S.A.), concentrated by differential density centrifugation

Abstract

A combination of separatory and analytical techniques was used to investigate the chemistry of 3 major elements and 9 trace elements in fine-grained Colorado oil shale. The separatory procedure involved low-temperature ashing, followed by high-speed centrifugation in a liquid density-gradient column. The density gradient ranged from 2.0 to 2.9 g cm −3. Application of this combination of techniques to four samples of oil shale demonstrates the feasibility of separating micrometer-size mineral grains held tightly by carbonates and organic matter. We have defined the “maximum enrichment factor” as being the ratio of the maximum observed concentration of an element in a given density fraction of ashed sample to the concentration of the same element in the bulk ashed sample. Resulting values of the maximum enrichment factors range from 1.2 to 12 for various elements. Hierarchial cluster dendrograms reveal different mineralogical residences for the elemental groupings of: Ca-Fe-Mn; Cu-Pb-Ni; As-Zn; and Mo-F-K-Fe. Fluorine is concentrated in the 2.4- and 2.5-g-cm −3 fractions, in association with illite.