Structural, hyperfine, and magnetic evolution of a natural magnetite–serpentine composite: Effects of magnetic separation and thermal treatment

In this work, the structural, hyperfine, and magnetic evolution of a natural magnetite–serpentine composite was systematically investigated in its as-collected state, after magnetic separation, and following thermal treatments. X-ray diffraction combined with Rietveld refinement reveals a lizardite-dominated matrix containing magnetite as the primary magnetic phase. Mössbauer spectroscopy identifies Fe2＋ and Fe3＋ cations distributed over distinct octahedral and tetrahedral sites in both lizardite and magnetite, providing insight into cation distribution and oxidation state evolution. Magnetic measurements reveal ferrimagnetic behavior dominated by magnetite, with a saturation magnetization of ∼79 emu g−1 at room temperature. The transition temperature was estimated from the derivative of the ZFC–FC magnetization curve with respect to temperature, which exhibits a relatively broad peak associated with the Verwey transition (118 K). This feature suggests the presence of relatively preserved magnetite within the ultramafic matrix, although the broad character of the transition may reflect structural disorder, particle-size effects, or magnetic interactions. In addition, the magnetic data indicate the coexistence of superparamagnetic behavior and possible spin-glass-like contributions, likely associated with surface disorder and nanoscale magnetic interactions. Water-assisted magnetic separation enhances the magnetic fraction, confirming that magnetite particles are embedded within the lizardite matrix. Thermal treatments at 500 °C and 1000 °C induce progressive dehydroxylation, oxidation of iron-bearing phases, and the formation of hematite, forsterite, silica, and clinoenstatite, leading to significant modifications in both hyperfine parameters and magnetic behavior. The results establish clear structure-hyperfine-magnetic correlations and demonstrate how magnetic separation and thermal processing govern the phase stability and functional magnetic response of natural magnetite–serpentine systems.