Potassium (K-) feldspar sand grains from Little Pipestone Creek 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 were detected by: 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. However, the (131)-(131) method reveals more about the true structural state and mode of origin of the K-feldspar grain. K-feldspar structural-state provinces were most effectively delineated by the combination of the optical and the (131)-(131) XRD methods. 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, this combined optical and X-ray approach should be even more effective.--Modified journal abstract.