Chemical Weathering Rates and Mineralogical Characteristics of Soils Developed on Tuff from Semilir Formation

Abstract

The Semilir Formation is part of the southern mountainous zone south of central Java. This formation consists of tuff, lapily tuff, lapily pumice, pumice breccia, and shale. The thickness of this formation is more than 460 meters. Studies on the chemical and mineralogical properties of soil derived from the weathering of tuff rocks in the Semilir Formation have never been carried out. Twelve soil samples from different areas within the Semilir Formation were analyzed for their chemical and mineral properties. X-ray diffraction (XRD) and X-ray fluorescence (XRF) techniques were used for chemical and mineralogical analysis. The results of the XRF test will be used to calculate an efficient chemical weathering index to assess the development of soil from the underlying rock. The XRD test results indicate that the dominant mineral contents in the samples are metahalloysite, quartz, plagioclase feldspar, and smectite. The chemical composition results show that the SiO₂, Al₂O₃, and Fe₂O₃ content in the residual tuff soil increases with the degree of weathering while CaO decreases. Furthermore, MgO, Na₂O, K₂O, and TiO₂ do not report a good correlation with the degree of weathering. This study demonstrated that, among the evaluated weathering indices, the Weathering Index of Parker (WIP), the Chemical Index of Alteration (CIA), the Chemical Weathering Index (CIW), and the Plagioclase Alteration Index (PIA) were the most proper indices for predicting tuff weathering intensity. CIW and CIA show very good correlations since CIW is a modified chemical weathering index from CIA; hence, the calculation of both indices shows similar results. WIP and CIA, WIP and CIW, and WIP and PIA also show good correlations.