Quantifying Internal Stress and Demagnetization Effects for Natural Multidomain Magnetite and Magnetite‐Ilmenite Intergrowths.

Demagnetizing effects and internal stress are difficult to distinguish in natural magnetite samples, but quantitative stress estimates can provide valuable information about microstructure formation, surface oxidation, impacts, tectonic stresses, or interface properties in exsolution structures. Quantifying demagnetizing effects informs about magnetite particle shape, magnetostatic interaction, or anisotropic texture. Here, we establish an improved measurement workflow to separate demagnetizing effects from internal stress for natural magnetite. The method is based on temperature‐dependent hysteresis measurements, and for natural samples require accurate estimates of Curie temperature and temperature‐dependent saturation magnetization to ensure that near‐end‐member magnetite is the dominant magnetic mineral, and to calibrate the temperature‐dependent scaled reversible work (SRW). SRW is the fundamental quantity to determine stress and demagnetizing factor. The improved SRW method is applied to three natural samples with different stress histories where it proves that large magnetite crystals in the metamorphosed Modum complex (Norway) have low internal stress (<100 MPa), while in highly exsolved magnetite‐ilmenite intergrowths from Taberg (Sweden) and Bushveld (South Africa) the magnetite component is highly stressed (>230 MPa). This confirms experimentally that interface strain in complex microstructures due to spinodal decomposition and partial oxidation creates large average internal stress in the magnetite minerals. Because sister specimens have similar internal stress but noticeably (>20%) different demagnetizing factors, textural, and shape anisotropy contribute substantially to SRW in these samples. Plain Language Summary: Magnetite is one of the most abundant magnetic minerals in nature and an important magnetic recorder over the entire history of the Earth. In natural rocks even chemically pure magnetite particles vary considerably in size, shape, and internal stress. Hitherto, mainly the variation in size has been systematically studied, partly because there was no experimental way to reliably quantify stress and shape effects. We now use a new technique to quantify stress and shape effects in three natural magnetite mineral types. One which formed millions of years ago at elevated temperature shows very little shape and stress influences. In the other two samples, slowly cooled magnetite formed a very fine network of lamellae at nanometer scale. In these cases, it was theoretically predicted that internal stress should reach values of up to 300–600 MPa at the interface. For the first time, we can experimentally verify that the high average stresses of up to 340 MPa indeed occur in these exsolution structures. The successful test of our new method extends the toolbox of rock magnetism by a valuable additional technique to quantify previously unknown internal properties of natural magnetite. Key Points: An experimental workflow is presented to separate stress and demagnetizing energy in natural magnetite samplesDemagnetizing energies in synthetic and natural magnetite have equivalent demagnetizing factors of ΔN = 0.33 − 0.46Average internal stress in natural nanoscale magnetite‐ilmenite exsolution structures corresponds to 230–340 MPa [ABSTRACT FROM AUTHOR]