Magnetic properties of Geraisites, the first tektite strewn field identified in Brazil

Geraisites are a newly discovered impact-related glass from Brazil. They occur as centimeter-scale, elongated to subspherical bodies scattered across surface gravel and shallowly buried layers in a strewn field located between the municipalities of São João do Paraíso and Curral de Dentro, close to the border between the states of Minas Gerais and Bahia (Brazil). Their magnetic properties show weak bulk magnetization dominated by a paramagnetic component, which is typical of tektites, but with a significant proportion of ferromagnetic particles. Their remanence is complex, and the decomposition of isothermal remanent magnetization curves reveals two magnetic components: a low-coercivity family with a mean coercivity of ~40–70 mT (DP = 15–20 mT) attributed to magnetite, and a high-coercivity family with coercivities exceeding 1 T attributed to goethite. The natural magnetization intensity varies between 1.06×10⁻⁶ Am²/kg (specimen 30) and 2.32×10⁻⁷ Am²/kg (specimen 43). Lowrie–Fuller tests reveal predominantly PSD–MD magnetite, especially in specimens 30 and 51, which exhibit ARM–IRM relations characteristic of PSD grains. Compared to other know tektites, typically characterised by extremely low magnetic mineral content, simple near-single-component NRM, and MDF values commonly below ~40 mT, the geraisites display stronger, more heterogeneous magnetic signatures and multiple coercivity components. In addition, geraisites exhibit a different relationship between magnetic susceptibility and FeO% content compared to other tektites, reflecting their higher ferromagnetic contribution and likely indicating a higher degree of oxidation. In some samples, a high initial remanence is followed by a sharp decay in remanence during AF demagnetization (specimens 30 and 43), suggesting overprinting by transient high-field processes, such as lightning. Collectively, these results indicate that geraisites host a mixed assemblage of magnetite, oxidized magnetite, and goethite whose relative proportions likely reflect both impact-related thermal conditions and later overprinting events.