Clay Mineralogy of the Meade Peak Member of the Phosphoria Shale Formation, Idaho

Characterization of clay mineralogy and age of illitic clay in samples from the Meade Peak Phosphatic Shale Member of the Phosphoria Shale Formation located in the Western Phosphate Field, SE Idaho, USA, are presented. Illite is the predominant clay mineral within these samples, with smectite, apatite, and 1:1 clays also found. A subset of the samples are further analyzed for illite-smectite (I-S) relationships. These samples were fractionated to <2μ m size. The half-height of the illite 001 peak range from 0.36^o to 0.61^o 2Θ . Srodon intensity ratios are in the range of 1.02 to 1.33. This results in XRD scattering domain size, N, of approximately 15 with 1-2 percent of swelling layers for I-S. The illite 002/smectite 003 reflection peaks of the EG-solvated patterns are approximately 5 Å for all samples and size fractions. This indicates that illite/smectite is composed of >90% illite. For the majority of the size fractions, a reflection peak around 8^o 2Θ is present, indicating that the clays exhibit mostly R3 ordering. In three samples, the K-Ar dates of the <0.25μ m size fractions are 209-252 Ma, the 0.25-1μ m size fractions are 281-290 Ma, and the 1-2μ m size fractions are 246-290 Ma. The decrease in K-Ar dates with decreased size fraction is consistent with an interpretation that the clay fraction is a mixture of detrital and diagenetic illite. A third generation of illite is seen in one sample of the Phosphoria as thin seam of small (1 mm) illite crystal rosettes.