Kauai Island is the northern most island of the Hawaiian Island chain. It is geologically the oldest of the major islands and is said to be the most beautiful. The island is also marked by a mineralogical diversity in beach sands. Kauai beaches are noted for a significant contrast between volcanic "black" beach sands and carbonate "white" beach sands. Samples of the sands were collected from eight different sites along the northern, southern, and western shorelines. Three more samples were collected of representative end members (homogeneously volcanic to homogeneously carbonate). The objective of this study was to determine if X-ray diffraction could effectively quantify the mineral assemblage All samples were analyzed on a Philips Analytical X-ray Diffractometer (PC-APD PW1710). After a preliminary qualitative analysis, a Rietveld full pattern refinement program was used to quantify mineral concentrations. Mineral assemblages for both volcanic and carbonate beach sands were complex. The carbonate beach sand components consisted of both high and low Mg calcite plus aragonite. Aragonite is less than 50% of the carbonate component, which reflects the dominance of coralline algae in Kauai Reefs. The volcanic beach sand component consisted of forsterite, magnesite, magnetite, plagioclase feldspar (both end members including intermediate phases), chlorite, ilmenite, pyroxenes, goethite, and hematite. Rietveld X-ray diffraction technique has advantages in that it takes in account and corrects for deficiencies in the pattern such as orientation preferences and poorly crystalline phases. Its disadvantages lie in the fact that it is necessary to define all phases present, which may be difficult when dealing with complex assemblages such as weathered volcanic material. It also has the advantage in that, in addition, to determining the relative proportion of volcanic and carbonate components, it yields detailed mineralogical information about end member components.