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3.2 e. Magnetic properties of P21/c (Mg,Fe)SiO3 pyroxene (C.A. McCammon in collaboration with S. Eeckhout, E. De Grave and R. Vochten/Ghent and A. Lougear, M. Gerdan and A. Trautwein/Lübeck)

Ca-poor pyroxene close to the join MgSiO3-FeSiO3 is an important phase in the Earth's upper mantle. Hence knowledge of its physical and chemical properties is crucial in modelling the mineralogy and dynamics of this region. Previous research, including many projects undertaken at Bayerisches Geoinstitut, has produced phase diagrams for the system showing a transformation of the P21/c structure to the C2/c structure with increasing pressure at moderate temperatures (Annual Reports 1994 and 1995). Such a transformation could be relevant to the dynamics of subducting slabs, and has motivated detailed studies of the crystal structure and properties of both P21/c and C2/c (Mg,Fe)SiO3 clinopyroxenes.

Magnetic properties provide information on the magnetic response of a particular mineral, and also on the nature of the linkages joining the magnetic atoms. In particular, comparison of the magnetic properties of different (Mg,Fe)SiO3 pyroxene polymorphs provides data complementary to single-crystal refinements. We therefore undertook a project to study the magnetic properties of (Mg,Fe)SiO3P21/c clinopyroxenes that were synthesised in a multianvil press at Bayerisches Geoinstitut and examined using Mössbauer spectroscopy (0.3 - 300 K) and SQUID magnetometry. Results were compared to those for (Mg,Fe)SiO3 orthopyroxene (space group Pbca), the only other polymorph for which detailed magnetic data are available.

Mössbauer spectra taken above and below the magnetic ordering transition show that the onset of magnetic ordering occurs at only one temperature for a given composition, indicating that similar to orthopyroxene, the Fe moments on the M1 and M2 octahedral sites are magnetically coupled. The actual magnetic transition temperature was determined using Mössbauer thermoscanning measurements, where the source is held at constant velocity and the count rate is determined as a function of temperature. Results indicated that magnetic transition temperatures for P21/c clinopyroxenes are similar to those for orthopyroxenes for a given composition.

Mössbauer spectra were collected at low temperature, and showed magnetic interactions that varied between different nearest-neighbour configurations. Spectra could be fitted assuming probabilities of each configuration based on Fe ordering onto the M2 site according to partition coefficients determined from the paramagnetic spectra, but an otherwise random distribution of Mg and Fe. Magnetic hyperfine parameters for P21/c clinopyroxene are close to those for orthopyroxene with the same composition, where the small differences are consistent with the slight variation in quadrupole splitting due to small differences in site distortion between the two polymorphs.

Measurements of the temperature dependence of the effective magnetic moment, the magnetic susceptibility and the field dependence of the mass magnetisation show mostly similar patterns to results for orthopyroxene, with the exception of the Curie-Weiss temperatures in the paramagnetic region. These temperatures are negative for compositions of P21/c pyroxene where xFe < 0.9, indicating that antiferromagnetic interactions dominate, whereas Curie-Weiss temperatures are positive for similar compositions of orthopyroxene, where ferromagnetic interactions dominate. Previous neutron diffraction measurements of orthopyroxene have shown that magnetic moments are ferromagnetically ordered within ribbons of zigzag chains of M1 sites, which are sandwiched between two linear chains of M2 sites, while collinear antiferromagnetic ordering occurs between the ribbons. Based on the similarity between the crystal structures of orthopyroxene and P21/c clinopyroxene, it is reasonable to assume that the magnetic structures are also similar. Hence the difference between magnetic ordering could be related to the competing effects of magnetic interactions within ribbons versus those between them. This likely reflects differences in the geometry of the SiO4 tetrahedral chains between the two structures.

This study of magnetic properties has shown that the local environment of Fe atoms is similar in P21/c clinopyroxene and Pbca orthopyroxene, leading to similar results for methods that probe the local iron environment such as Mössbauer spectroscopy. In contrast, differences in interactions involving the silicate chains have a significant effect on the bulk properties of the mineral, in this case the magnetic behaviour. While differences in magnetic properties between (Mg,Fe)SiO3 polymorphs are unlikely to have any direct geophysical implications due to the temperatures involved, they do emphasise that small differences in crystal structure can lead to large differences in bulk behaviour in cases where competing effects (in this case ferromagnetism and antiferromagnetism) are finely balanced.

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