• February 9, 2023

Research Paper released on Quantifying weathering intensity using chemical proxies: a weathering index AFB

Published on February 9th, 2023

CSA Global congratulates Senior Geologist and author, Dr Nikita Sergeev on his recently published paper: ‘Quantifying weathering intensity using chemical proxies: a weathering index AFB” published by the Australian Journal of Earth Sciences, an International Geoscience Journal of the Geological Society of Australia.


Chemical weathering indices (CWIs) based on bulk chemical rock composition can potentially provide an objective tool for estimation of weathering intensity and classification of weathering products. However, despite their long history and the large number of indices that have now been developed, their applicability still has serious limitations. To overcome the limitations, the paper proposes a new weathering index based on the review of geochemical and mineral rock evolution during weathering and analysis of the existing CWIs.

The new index, the Alumina + Ferric Oxide to Bases ratio (AFB), is expressed as

AFB = AFBw/AFBp where AFB = (Al2O3 + Fe2O3)/(K2O + Na2O + CaO + MgO), w – weathered rock and p – parent rock, with all elements in molecular proportions.

The index provides a basis for comparison of weathering intensities between different lithologies by linking the chemical and mineral transformations that characterise the regolith profile. It is sensitive to all stages of weathering, including lateritisation by using the ratio of the stable (relatively immobile) to mobile metals expressed as oxides. An extensive, world-wide chemical database on 41 well-documented regolith profiles developed on felsic, mafic and ultramafic substrate was used for testing the proposed index and its comparison with the most common indices. The results confirmed consistent increase in the AFB value with intensifying weathering. The index is applicable to all silicate rock types including Al-poor ultramafic rocks. Use of parent rock normalisation allows a more accurate comparison of weathering intensities between different lithologies. At the expense of these benefits, the parent normalised index AFB requires data for the unweathered protolith which is often difficult to obtain. The index is also sensitive to inhomogeneity of the original and weathered rocks. The unreferenced to parent rock AFBu index has potentially broader applications including provenance and the weathering history of sediments, soil and engineering studies although collection of more data is required for understanding the index constraints for various conditions and rock types.

About the Author

Senior Geologist, Nikita Sergeev
Dr Nikita Sergeev
Senior Geologist

Nikita is an exploration geologist and geochemist with over 25 years’ multi-commodity and global experience across a variety of geological terranes. He has has a broad exploration background (ranging from grassroots reconnaissance through to brownfields, near-mine and resource definition), with particular expertise in exploration geochemistry, spectral mineralogy and regolith interpretation. He has managed and provided technical geological expertise for numerous exploration projects for a wide range of mineral commodities across Australia and overseas. Nikita has experience in environmental geochemical site assessments and monitoring including acid mine drainage assessment of mine tailings around several VMS Cu-Zn deposits in the Urals, Russia, and  geochemical monitoring of iron mines in Western Australia.

Link to article: Quantifying weathering intensity using chemical proxies: a weathering index AFB

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