Magnetite originating from bonfires in a Brazilian prehistoric Anthrosol: A micro-Raman approach

Abstract Soils within archaeological sites may show magnetic properties due to the presence of magnetic minerals, such as magnetite and maghemite, which form during deposition of the soil or due to local fires. The similarities in the crystallographic structures of magnetite and maghemite lead to difficulties in the identification of these mineral phases. This work aims to study the genesis of Fe nodules at an archaeological site in a cave in quartzite with a very low Fe content and no spinel group minerals. The guiding hypotheses of the research are as follows: i) the abundant Fe nodules in all the studied stratigraphic layers are of pedogenic origin and were generated by local bonfires, and ii) these Fe nodules are composed of magnetite, maghemite, and hematite, and the conversion of magnetite to maghemite indicates pedogenic processes. Micromorphological analysis and Raman spectroscopy were used for mineral phase determination. The results confirm the in situ pedological nature of the Fe nodules associated with anthropogenic fires in the cave.