Trioctahedral micas: relationships between crystal chemistry and magnetic behavior

This study is aimed at verifying the factors affecting magneticsusceptibility χ variation in micas in relation to: I) Fe content;ii) Fe2+/Fe3+ ratio; iii) occurrence of Fe in tetrahedral andoctahedral sites, outlining possible ordering effects.Other aspects considered in our study will involve: i)eventually observed variation in magnetic properties forsamples showing similar chemical composition, but occurringfrom different areas and different crystallization environments;ii) possible influence of strong magnetic anomalies overmagnetic properties of samples therein occurring.All these aspects were thus addressed by characterizingsamples belonging to phlogopite – annite, tetra-ferriphlogopite– tetra-ferri-annite and polylithionite – siderophylliteseries. Some samples, representative of the whole sample set,were then selected for magnetic susceptibility characterization.Some of them also required calorimetric analyses, in order tobetter describe and understand their behaviour.Other experimental methods used in this investigationinclude: electron microprobe, structure resolution by X-raysingle crystal methods, X-ray absorption spectroscopy, with aparticular attention to the understanding of the XANES region.Main results demonstrate that: i) magnetic phenomenaassociated with maxima of magnetic susceptibility could beobserved at low temperature (T<10 K) and weak magnetic field(10 Oe); ii) magnetic properties depend not only on the total Fecontent. Samples where Fe is mostly in octahedral coordinationare predominantly ferromagnetic, strongly depending on the[6]Fe2+/([6]Fe2++[6]Fe3+) ratio, which was observed to increasewith Curie-Weiss θ constant (ranging from 13 to 24 K and from4.4 and 5.2 K for samples showing higher and lower[6]Fe2+/([6]Fe2++[6]Fe3+) ratio, respectively). In tetra-ferriphlogopite,dominant interactions were observed to be antiferromagnetic,as shown by a negative Curie-Weiss θ constantvalue (-25 K). Furthermore Curie-Weiss constant (θ) and thetemperature value giving the maximum in the magneticsusceptibility (Tm) seem to be dependent not only on the meannumber of Fe atoms in octahedral coordination (Z) per unit cell,but also on its ordering. Tm linearly depends on severalstructural parameters as well, e.g., the tetrahedral rotation angle α.