The Chemical and Mineralogical Characteristics of Quaternary Volcanic Rock Weathering Profile in the Southern Part of Bandung Area, West Java, Indonesia

Upstream Citarum river basin is located in the southern part of Bandung Regency. This region is composed by Quaternary volcanic products. In general, the exposed rocks have been weathered with varying levels. Several studies have linked the chemical and mineralogical characteristic of the weathering process has been carried out, among them are Sukiyah et al (2000), Sukiyah et al (2005), Sukiyah (2009), Sukiyah et al (2010). The results showed that at certain weathering level will result soil with specific chemical and mineralogical characteristics as well. Nevertheless, the correlations among these variables have not been analyzed in detail. This study aimed to determine changes in the chemical and mineralogical of volcanic rocks at different levels of weathering and the correlation between them. Volcanic rocks in the study area can be grouped in alterated and unalterated rocks. Major oxides obtained from 33 rock samples with varying degree of weathering. The chemical characteristics data (major oxides) obtained by XRF methods performed in the laboratory. Meanwhile, the minerals data obtained through petrographic analysis using the polarizing microscope, XRD, and SEM. The probabilistic approach is used to analyze the data to determine the chemical and mineralogical characteristics at different levels of weathering. The analysis showed that major oxides proportion of altered volcanic rocks not influenced by the degree of weathering of rocks is significantly. Otherwise, the degree of weathering strongly correlate with the proportion of the major oxides in unaltered volcanic rocks. In unaltered volcanic rocks, SiO2, Al2O3, Fe2O3, CaO, Na2O, K2O, and TiO2 form a significant chart patterns, contrarily P2O5 and MnO did not show any correlation. Increasing the degree of weathering resulted in a decrease in the SiO2, CaO, MgO, Na2O, and K2O, instead Al2O3, Fe2O3, and TiO2 increased significantly. In altered volcanic rocks, correlation between Al2O3 and P2O5 on LOI is not significant. In contrast, SiO2, Na2O3, and K2O have a significant correlation. Meanwhile, changes in Fe2O3, CaO, MgO, TiO2, and MnO to the degree of weathering is fluctuating. In general, volcanic rock-forming mineral susceptible to weathering processes. In line with the increase in the degree of weathering, the cavity and iron oxide increased as well. At the advanced weathering, volcanic rock forming mineral have been altered entirely due to chemical reactions. Al3+and Fe3+ are cation most vulnerable regardless when the weathering process begins. Increasing degree of weathering, the other cations also separated to form a new compound. They are more stable with the degree of weathering conditions. Based on USCS classification, soil formed belonging to the high plasticity silt (MH). Clay minerals dominated by halloysite and kaolinite. Halloysite is derived from the weathering of feldspar.They are found in soil of volcanic rocks weathering results. Kaolinite are relatively resistant to weathering, but can turn into gibsite if the silica content regardless.