ABSTRACT

Natural radioactivity was measured in two series of Wilcox (Eocene) lignites from Rusk and Panola Counties, Texas. Natural radioactivity consists mostly of ²³⁵U, ²³⁸U, ²³²Th decay chain isotopes, and ⁴⁰K. Uranium isotopes occur in lignite in two ways: in heavy mineral crystals carried by streams and deposited with the lignite as it forms, and trapped in the complex molecules of organic matter in the lignite. Thorium occurs only in the heavy minerals in a known concentration ratio with uranium. Potassium occurs in association with the plant remains of the lignite.

A total of 31 samples were analyzed by gamma-ray spectrophotometry for uranium, thorium, and potassium. Uranium averages 1.18 ppm U and ranges from 0.91 to 1.37 ppm U. Thorium averages 7.32 ppm Th and ranges from 6.30 to 8.87 ppm Th. Potassium averages 0.55 weight percent K and ranges from 0.51 to 0.61 weight percent K. The Th/U ratio of 5.5 is within the range of common heavy minerals like zircon, monazite, and allanite, showing that there is little organic uranium present in these lignites. If organic uranium were present, the Th/U ratio would be less than about 3 to 4.

Wilcox lignite is lower in uranium than the average United States lignite, and strikingly lower than lignite from South Texas where the sediment enclosing the lignite is a low-level source of uranium. In contrast, East Texas uranium-rich sediments are some 80 mi south of the Wilcox lignites.