High efficiency of natural lamellar remanent magnetisation in single grains of ilmeno-hematite calculated using Mössbauer spectroscopy
Benjamin P. Burton, S. McEnroe, C. McCammon, P. Robinson
Single grains of ilmeno-hematite (titanohematite with ferri-ilmenite exsolution lamellae) approximately 250 µm in diameter were taken from Mid-Proterozoic metamorphic rocks in southwest Sweden and the Adirondack Mountains, USA, and were studied at room temperature using Mössbauer spectroscopy. Mössbauer spectra of synthetic samples of titanohematite (Fe0.95Ti0.05O3 and Fe0.9Ti0.1O3) were collected for comparison, and showed a dominant six-line magnetic spectrum due to Fe3+ in titanohematite with a weak sextet due to Fe2+-Fe3+ charge transfer. Mössbauer spectra of the natural ilmeno-hematite grains were similar to those for synthetic titanohematite, but contained additionally two paramagnetic doublets corresponding to Fe2+ and Fe3+ in ilmenite, and in several grains there was an additional weak, broad magnetic component that we assigned to iron in contact layers according to the lamellar magnetism model. Similar to previous Mössbauer results for natural hemo-ilmenite, there was no evidence for superparamagnetic behaviour of the nanoscale titanohematite, and no evidence for single-domain or superparamagnetic magnetite. The compositions of titanohematite and ferri-ilmenite in the individual ilmeno-hematite grains were calculated from the relative area ratios of the Mössbauer subspectra, and results showed that compositions were closer to endmember values compared to electron microprobe and transmission electron microscopy data, and consistent with observations that lamellae widths are as small as a few nm. Bulk compositions of ilmeno-hematite grains calculated from the Mössbauer data showed that the Swedish samples are significantly more ilmenite-rich than the Adirondack samples, and that results are consistent with an estimation of bulk composition through point counting of one of the electron backscattered images. The identification of iron in contact layers by Mössbauer spectroscopy enabled the first ever estimation of contact layer abundance, which was found to be roughly 8% by volume for a bulk composition of (FeTiO3)36(Fe2O3)64. This abundance effectively controls the magnetisation, but so far has not been able to be determined by any other means.
, McEnroe, S.
, McCammon, C.
and Robinson, P.
High efficiency of natural lamellar remanent magnetisation in single grains of ilmeno-hematite calculated using Mössbauer spectroscopy, Earth and Planetary Science Letters, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=901408
(Accessed February 29, 2024)