Several studies have addressed the elaboration of petrographic indices, applicability, and their execution in the characterization of weathering grades. Petrographic indices directly depend on a visual criterion used to distinguish fresh mineral from weathered mineral, which is usually not shown. As investigated, this causes a lack of coherence among petrographic indices data for mechanical properties estimation aiming at engineering geology applications. This paper involved a readjustment of technique for the calculation of weathering indices and the establishment of criteria to gather the minerals under weathering into the petrographic index's formula based on a standardized classification of textural and micromorphological modifications. The results have shown that varying the criteria influence indices values for the same physical–mechanical properties, resulting in different types of best-fit lines, but with reasonable coefficients of determination along the weathering grades. This in part explains the variability of literature correlations. The use of a standardized classification of rock-forming minerals contributed to finding closely related mechanical degradation tendencies among literature data.
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We thank CNPq (National Council for Scientific Research) for the scholarship and the Department of Sedimentary and Environmental Geology of the Institute of Geosciences and the Engineering School of the University of São Paulo, which made the development of this research possible. Special thanks to Ph.D. Rogério P. Ribeiro, from Department of Geotechnical Engineering (USP), for their comments during the development of this study, and to the journal reviewers for the comments which helped to improve the paper.
This study was funded by the National Council for Scientific Research (CNPq).
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Monticelli, J.P., Sígolo, J.B. & Futai, M.M. On weathering understanding and its characterization by petrographic indices: a study case about the criterion establishment for the textural classification of rock-forming minerals under weathering. Environ Earth Sci 80, 408 (2021). https://doi.org/10.1007/s12665-021-09687-3