ИСТИНА

Fe-rich layer silicates are rare on Earth, but are a major component of CM2 carbonaceous chondrites. They formed during early aqueous alteration events that affected CM2s’ asteroidal or cometary parent body [1]. Bulk magnetometry (300-2 K) has proven useful for characterizing Fe-serpentines’ mineralogy, close to the pole cronstedtite in CM2s [2]. Comparison with results from single crystals supports the hypothesis of a variable Fe content of serpentines along the alteration sequence of CM2s [2], similarly to terrestrial serpentinization. Site distribution and valence of iron in serpentines would directly be linked to their growth conditions (T, water-rock ratio or duration [3]). Understanding the conditions in which the reactions occurred on the parent body of CM2s thus relies on a thorough characterization of their crystal chemistry. Here we present the results of a X-ray Magnetic Circular Dichroism study at the Fe K-edge of oriented single crystals of cronstedtite, showing a strong planar anisotropy. We measured XMCD at various angles between the c axis and the applied field. We will show how ligand field multiplet calculations [4] allow one to separate various contributions to the pre-edge. This would yield an estimate of the crystal chemistry of Fe in this multisite (octahedral, Oh, and tetrahedral) and multivalent (Fe2+,3+) mineral, allowing for a fine characterization of this alteration mineral in meteorites. Also, XMCD suggests that the anisotropy of cronstedtite originates from the strong single ion anisotropy of Fe2+ in distorted Oh sites. Finally, exploring the variations of XMCD with substitutions (mostly Fe-Si) would clarify which parameters control the disruption of a long-range magnetic order in Fe-serpentines, from AF, when only Fe2+ is present, in Oh sites [5], to spin-glass like, in cronstedtite [2]. [1] Zolensky, Krot & Benedix (2008) Rev. Miner. Geochem. 68, 429-462. [2] Elmaleh, Tarantino, Zema, Devouard & Fialin (2012) Geochem. Geophys. Geosyst., 13, Q05Z42. [3] Marcaillou, Muñoz, Vidal, Parra, & Harfouche (2011) EPSL,303, 281-290. [4] Arrio, Rossano, Brouder, Galoisy & Calas (2000) Europhys. Lett., 51, 454. [5] Coey, Ballet, Moukarika & Soubeyroux (1981) Phys. Chem. Miner. 7, 141-148.