ABSTRACT

Potassium (K-) feldspar sand grains from Little Pipestone Creek and its tributaries within the Boulder Batholith of southwest Montana have been analyzed in order to evaluate the effectiveness of optical and X-ray techniques of K-feldspar structural-state discrimination for the definition of detrital mineralogical provinces and the interpretation of provenance.

Source-rock-controlled structural-state variations in the detrital K-feldspar in Little Pipestone Creek were detected by two methods: the optical determination of the percentage of cross-hatch (albite-pericline) twinning, and the (131)-(131) X-ray diffraction (XRD) determination of the obliquity of individual K-feldspar sand grains. The optical method is the more efficient; a much larger number of grains can be analyzed in a given period of time. However, the (131)-(131) XRD method has greater resolving power; each analysis reveals much more about the true structural state and mode of origin of the K-feldspar grain than does the mere presence or absence of twinning. The even more precise (060)-(204) XRD method for determining K-feldspar structural state was found to be ineffective in his application because of its much lower efficiency. Neither XRD method proved to be effective when applied to aggregate-grain sand samples.

K-feldspar structural-state provinces within this small, Holocene "sand body" were most effectively delineated by the combination of the optical determination of the percentage of cross-hatch twinning and the (131)-(131) XRD characterization of the structural state of selected individual K-feldspar sand grains. In larger, ancient sand bodies, particularly in complex alluvial sand systems where the feldspars have been derived from a varied array of feldspathic parent rocks (e.g., by the unroofing of plutons and their associated volcanic piles) this combined optical and X-ray approach should be even more effective.