Accurate information on the weathering degree of the ground composed of rocks is very important for ensuring the stability of large-scale buildings such as high-rise buildings, inclined slope, and underground structures such as tunnels. Therefore, many weathering indices (Chemical Index of Alteration;CIA, Chemical index of Weathering;CIW, Weathering Index of Plagiocalse;WIP, etc.) have been proposed in order to decide the weathering degree of rocks.
However, recent studies have shown that previous chemical weathering indexes of rocks can not accurately reflect weathering degree of rocks. This problem is caused by simply using whole rock compositional change for evaluating weathering degree without taking into consideration on the complex weathering process related to the change of mineral constituents and their compositions during weathering, which are important for evaluating the weathering process of rocks. Therefore, this study is carried out to suggest new weathering index using biotite which is less affected by the complex weathering process compared to the previous weathering indexes. targeting the Jurassic granite, which is the most widely distributed rock in Korea..
Based on the reference works, the Jurassic granitoids in the Korean peninsula are divided into 10 types. Among them, the biotite granite is distributed most widely in the Korean peninsula and other types of the Jurassic granitoids are abundant in the following order; the granodiorite. foliated granodiorite, foliated granite, porphyritic biotite granite, two mica granite, porphyritic granodiorite, foliated porphyritic granite, alkali granite and porphyritic foliated granodiorite. The biotite granite, granodiorite, strongly foliated granodiorite, porphyritic granite and porphyritic weakly foliated granodiorite are selected as five representative rock types for weathering experiments. Polarization microscopy observation, whole-rock analysis (XRF) and EPMA analysis are performed to study the change of minerals and their compositions according to the weathering degree. Based on polarization microscopic observation, the weathering degree of the rocks is divided into five grades according to the weathering degree of plagioclase and biotite. It is confirmed that the existing weathering indices using whole-rock data do not reflect the weathering degree of the five representative granitoids. As a result of quantitative analysis of biotite using EPMA, it is confirmed that the K2O value of biotite decreases systemically with increasing the degree of weathering. Therefore the Changwhan-Seunghyun Biotite Weathering Index (CSBWI) based on the change of the interlayer cation K value of biotite is suggested as a new weathering index for the Jurassic granitoids in the Korean Peninsula..
In this paper, the procedure for calculating the new weathering index, CSBWI, is proposed. The CSBWI is found to represent the weathering degree of the Jurassic granitoids better than the existing weathering indices and seems to have good relationship with the physical properties of the rocks, such as unconfined compression strength, suggesting the possibility of being used to determine physical properties. As biotite is a common mineral in metamorphic rocks as well as in igneous rocks, The CSBWI may be applied to evaluate the weathering degree of other various rock types.