Applicability of chemical indices to characterize weathering degrees in decomposed volcanic rocks

Among the useful tools in characterizing weathering degrees are chemical indices, which combines bulk major elemental oxides into a single metric value and commonly presented along a vertical profile. However, characterizing geomaterials of diverse nature using chemical indices may be problematic. In this study, chemical indices were evaluated to characterize weathering degrees in decomposed volcanic rocks from different depths, locations and formations. This was achieved by conducting comprehensive geochemical tests and statistical analyses. A good index must give appropriate trends, provide meaningful statistics and varies greatly with weathering degrees irrespective of the approach employed. Correlations were made between chemical indices and specific volume, depth, weathering degree as well as geological formation using tentative numerical value. The extent of weathering is best characterized by relating chemical indices with in-situ specific volume rather than depth, which is commonly used. The weathering potential index (WPI), hydration coefficient (Hc), leaching coefficient (Lc) and weathering index of Parker (WIP) are the most appropriate indices that could be used to characterize weathering in decomposed volcanic rocks. The chemical indices that have many mobile elements or cations and water content in their formulations are most useful. The chemical indices that are relating few oxides to each other should be avoided. The influence of formation on using chemical indices to characterize weathering degree is insignificant and the effect of weathering grade is very significant. This approach can be applied to similar geomaterials.