Compositional, thermal, and orientation dependency of olivine magnetic properties

Details Compositional, thermal, and orientation dependency of olivine magnetic properties Compositional, thermal, and orientation dependency of olivine magnetic properties

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusmgp21a..13b&link_type=abstract

American Geophysical Union, Spring Meeting 2007, abstract #GP21A-13

Physics

1518 Magnetic Fabrics And Anisotropy, 1519 Magnetic Mineralogy And Petrology, 1540 Rock And Mineral Magnetism, 3900 Mineral Physics, 5400 Planetary Sciences: Solid Surface Planets

Scientific paper

Olivine is an orthosilicate solid solution between forsterite Mg2SiO4 (Fo100) and fayalite Fe2SiO4 (Fo0). Cations such as Si4+ and Mg2+ are responsible for the diamagnetic behavior, whereas Fe2+ and other cations, present as defects in the lattice, such as Fe3+, Cr2+, and Mn2+, contributes to the paramagnetic behavior of the olivine. Yet, most natural and even synthetic olivines contain ferromagnetic iron oxide exsolutions similar to those commonly reported in other mafic silicates. Olivine is one of the most abundant constituents in the upper mantle of rocky planets, in meteorites, and in cosmic dust. Olivines with composition ranging form Fo100 to Fo0 are then exposed to temperatures that vary widely from about 5 K, away from the Sun in space, to about 1773 K at the 410 km olivine-wadsleyite transition. Below the Néel temperature (TN< 65 K), natural and synthetic fayalites exhibit a magnetic transition interpreted as the change from paramagnetic to antiferromagnetic behavior. A second low-temperature transition has been described around 20 K in fayalite and was attributed to a change from collinear to canted antiferromagnetic state. The magnetic properties of fayalite are also varying with crystal orientation. At decreasing temperature, just below TN, the magnetic susceptibility along the b axis stops following the Curie-Weiss law and begins to decrease. This drastic change is a clear indication of the antiferromagnetic behavior. The variation of magnetic susceptibility as a function of decreasing temperature along the two other axes, a and c, remains practically constant through the Néel transition. At the second low temperature transition (Tt), around 23 K, the variation of magnetic susceptibility with temperature changes along the c axis whereas no changes are noted along a and b. Experiments were performed on natural and synthetic ferromagnesian olivines of various iron contents between 4 and 300 K to further investigate their magnetic properties, regarding low-temperature transitions, compositional dependency, and magnetic anisotropy. Measurements were carried out in high magnetic fields, above the saturation of the ferromagnetic exsolutions to get access to the paramagnetic or diamagnetic properties of olivine.

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