Magnetic characterization of synthetic titanomagnetites: Quantifying the recording fidelity of ideal synthetic analogs

Abstract

A series of four synthetic basalts comprising titanomagnetite (Fe3‐xTixO4) grains of varied size and titanium content have been produced by a glass‐ceramic method. Complementary characterization techniques of X‐ray diffractometry, secondary electron microscopy, and transmission electron microscopy (TEM) demonstrate the reaction product composition consisted of mainly Fe3‐xTixO4, pyroxene hedenbergite, fayalite, and SiO2. The samples exhibit bimodal distributions of larger (<2 µm) and smaller Fe3‐xTixO4 particles (<50 nm in diameter), the latter found inside pyroxene crystals, as well as the sporadic occurance of dendritic Fe3‐xTixO4 structures. Magnetic measurements show their bulk characteristics fall into two groups: Ti‐rich titanomagnetite samples with varying Ti content; and near‐stoichiometric magnetite. The TEM technique of off‐axis electron holography allowed for visualization of the magnetic behavior of the synthetic Fe3‐xTixO4 grains. Energy dispersive X‐ray analysis and off‐axis electron holography confirmed the small Fe3‐xTixO4 grains (<50 nm) confined within glassy pyroxene regions to be Fe‐rich and single domain, carrying strong magnetic signals, compared to the relatively magnetically weak larger Fe3‐xTixO4 grains (x ∼ 0.6). The large grains in the pure magnetite sample are shown to be pseudo‐single domain in nature. The quenching process involved in synthesis is considered similar to that of pillow basalts found at mid‐ocean ridges and hence the reaction products are thought ideal in terms of characterization and understanding, for the purpose of studying natural systems.