Magnetism of micas: a comparison between crustal chemistry and AC susceptibility

Although silicate minerals are the principal constituents of Earth’s crust and mantle and iron is the most widespread magnetic element, few reliable studies on magnetic properties of iron-silicates were performed on natural samples, chemically and structurally well characterized. This work will address layer silicates magnetic properties, with special focus on micas, which, from a magnetic viewpoint, can be classified as bidimensional systems.Iron in micas can be present as Fe2+ and Fe3+ and thus different magnetic properties can arise from their 3d6 and 3d5 electronic configuration. Furthermore, differences in magnetic properties can be related to coordination. In fact Fe3+can substitute Si4+ in tetrahedral coordination in the phlogopite – tetra-ferriphlogopite join whereas Fe2+can substitute Mg2+ in octahedral coordination in the tetra-ferriphlogopite – tetra-ferri-annite join.Moreover, to preserve layer charge neutrality, other different mechanism can be introduced to compensate, for example, Fe3+ octahedral content.In this work, we considered four natural mica samples, two belonging to the phlogopite – annite join (Tag15-4 e C6b), one to the tetra-ferriphlogopite – tetra-ferri-annite join and one to the polylithionite – siderophyllite join.Crystal-chemical data suggest differences in the amount of Fe2+ in octahedral coordination between samples Tag15-4 and C6b ([6]Fe2+ 15 e 50% respectively).Tetra-ferriphlogopite Tas22-1 is characterized by Fe3+ tetrahedral substitution and by small Fe2+ and Fe3+content in octahedral position. Sample 103 shows Fe2+ and Fe3+in octahedral position.The susceptibility-temperature curves, related to AC magnetic measurements in thetemperature range 77-298 K, suggest an antiferromagnetic behaviour for Tag15-4,C6b and Tas22-1, whereas the susceptibility-temperature of sample 103 is completelydifferent.The different behaviour of sample 103 can be related to structural disorder that prevents Fe atoms alignment in octahedral sites.Moreover the magnetic magnitude is similar for C6b and Tag15-4 but differs substantially in Tas22-1, probably because of iron oxidation state and occupancy in the layer structure: mainly Fe2+ in octahedral sites for C6b and Tag15-4 and Fe3+ in tetrahedral sites for Tas22-1.