Quantitative Mineral Identification of Alluvial Soils Using the Rietveld Refinement: Sequential vs. Batch Refinement

Information of mineral phases in soils is essential for understandng physicochemical characteristics of soils in unsaturated zone. In this study, mineral identification and quantitative phase analysis were performed for alluvial soils by Rietveld refinement with X-ray powder diffraction (XRPD) data. For systematic analysis, a grid soil sampling method was adopted for the target area of 100 m width and 80 m length at Deokso, Gyeonggi Province. Core samples for soils were collected at intervals of 20 m with a depth of 2 m by a geotechnical drilling rig. For the XRPD analysis, clay size particles were only prepared by a wet sieving and an oven drying. In the process of refinement, two different approaches were applied to determine which approaches yield more quantitative results. One is to increase the number of constituent minerals step by step after optimization for a single mineral. The other is to perform a batch optimization on all constituent minerals at once. As a result of the refinements, the weighted residual square sum (Rwp) of less than 10% is obtained on average in both cases, indicating a reliable result. Further significance test revealed that these results are indistinguishable. Although the both approaches are statistically acceptable for a set of mineral phases, the spatial abundances of individual minerals are not consistent with each other. This would show that goodness of fit is independent from getting the correct solution. Therefore, it is necessary to cross a check with the X-ray fluorescence data.