Applications of Mössbauer Spectroscopy to Lamellar Magnetism

Nanosize lamellar structures that form in slowly cooled igneous and metamorphic rocks can have unusually high natural magnetic remanence that is stable over timescales of one billion years or more. This behavior has been attributed to lamellar magnetism in the system hematite-ilmenite, where ferrimagnetic contact layers form between paramagnetic ilmenite and antiferromagnetic hematite. Such stable magnetic memory is of importance for planetary magnetism, particularly for planets such as Mars where the magnetic field is no longer present, and for potential industrial applications such as the development of highly stable magnetic storage units. Mössbauer spectroscopy provides a unique probe of the iron environment, enabling the quantitative determination of iron distribution and abundance, with insight into parameters that determine the magnetic properties. To investigate the role of iron in lamellar magnetism, we have undertaken a study of rocks from several different regions using Mössbauer spectroscopy. Titanohaematite grains were identified optically on polished thin sections of slowly cooled rocks from Lerhuvud and Gödestad (both in southern Sweden) and the Russell Belt (Adirondack Mountains, USA). Grains were removed from thin sections with a microdrill, and mounted on a Mössbauer spectrometer fitted with a point source. Room-temperature Mössbauer spectra are dominated by magnetically ordered Fe3+ in hematite, with a smaller absorption corresponding to paramagnetic Fe2+ in ilmenite. Minor absorption is also observed from magnetite and pyrite in some grains. There is no evidence for superparamagnetic hematite in any of the spectra. Comparison of Mössbauer spectra of the natural samples with those from synthetic hematite-rich titanohematite solid solutions provides a measure of the iron environment in natural titanohematite, showing that there is only moderate deviation from the ideal hematite local environment. The absence of Fe3+ in ilmenite indicates that ilmenite lamellae are close to the endmember composition. All grains taken from the same thin sections show similar ilmenite:hematite area ratios, and the two different samples from Gödestad also show similar ratios, suggesting a similar bulk composition. Based on models of cation and magnetic ordering, the proportion of iron involved in the contact layers can be determined, and combined with information from the Mössbauer spectra, provide insight into the density of lamellae, which effectively controls the magnetization.