Chapter 7 The effects of weathering on the mineralogy of the phosphoria formation, Southeast Idaho

Abstract The Permian Phosphoria Formation of the western United States is one of the largest phosphate deposits in the world. Despite the economic significance of this formation, its fine-grained nature has discouraged detailed mineralogical characterization and quantifi- cation studies. Recently, selenium and other potentially hazardous trace elements in mine waste rock have drawn increased attention and motivated extensive studies. Part of this effort has focused on a more detailed geological and mineralogical characterization of the rocks. This study uses powder X-ray diffraction (XRD) with Rietveld quantification soft- ware to quantify and characterize the mineralogy of samples collected from nine measured stratigraphic sections from the Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation at four active phosphate mines, all near Soda Springs, Idaho. These measured sections include four pairs of more-weathered and less-weathered sections as well as a single, deep, least-weathered section. Mineralogical analyses of samples from these nine sections reveal how weathering has affected the mineral content, identifies the minerals that host the trace elements of environmental concern, and discusses the complex variations in the composition of the carbonate fluorapatite (CFA). Carbonate minerals decrease sharply in concentration with increased weathering, as do pyrite and sphalerite. Because these sulfide minerals have been linked to trace elements such as Se, their weathering likely contributes to the geochemical cycling of these trace elements. CFA occurs in phosphorites globally, with varying amounts of carbonate substi- tuted for phosphate in the apatite structure. Analysis of the measured sections shows strong connections between the variable degree of carbonate substitution in the CFA structure and the host rock type. However, there is also evidence that weathering plays an important role in the composition and structure of the CFA, apparently breaking it down into nondif- fracting CFA-like compounds.