Pyroxenes with the general formula AM Si 2 O 6 (A = mono-or divalent metal, M = di-or trivalent metal) are shown to be a new class of multiferroic materials. In particular, we have found so far that NaFeSi 2 O 6 becomes ferroelectric in a magnetically ordered state below 6 K. Similarly, magnetically driven ferroelectricity is also detected in the Li homologues, LiFeSi 2 O 6 (T C 18 K) and LiCrSi 2 O 6 (T C 11 K). In all these monoclinic systems the electric polarization can be strongly modified by magnetic fields. Measurements of magnetic susceptibility, pyroelectric current and dielectric constants (and their dependence on magnetic field) are performed using a natural crystal of aegirine (NaFeSi 2 O 6 ) and synthetic crystals of LiFeSi 2 O 6 and LiCrSi 2 O 6 grown from melt solution. For NaFeSi 2 O 6 a temperature versus magnetic field phase diagram for NaFeSi 2 O 6 is proposed. Exchange constants are computed on the basis of ab initio band structure calculations. The possibility of a spiral magnetic structure caused by frustration as origin of the multiferroic behaviour is discussed. We propose that other pyroxenes may also be multiferroic, and that the versatility of this family offers an exceptional opportunity to study general conditions for and mechanisms of magnetically driven ferroelectricity.

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Molten I-type cosmic spherules formed by heating, oxidation and melting of extraterrestrial Fe,Ni metal alloys. The entire oxygen in these spherules sources from the atmosphere. Therefore, I-type cosmic spherules are suitable tracers for the isotopic composition of the upper atmosphere at altitudes between 80 and 115 km. Here we present data on I-type cosmic spherules collected in Antarctica. Their composition is compared with the composition of tropospheric O2. Our data suggest that the Earth's atmospheric O2 is isotopically homogenous up to the thermosphere. This makes fossil I-type micrometeorites ideal proxies for ancient atmospheric CO2 levels.