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371.
G. P. Bernardini D. Borrini A. Caneschi F. Di Benedetto D. Gatteschi S. Ristori M. Romanelli 《Physics and Chemistry of Minerals》2000,27(7):453-461
An EPR and SQUID magnetometry study of Cu2FeSnS4 (stannite) and Cu2ZnSnS4 (kesterite) has been performed in order to gain a deeper insight into the crystal chemistry of these minerals, in which the
mixed character of bonds lends uncertainty to the determination of the metal valence states. EPR investigations were performed
down to almost liquid nitrogen temperature on both natural and synthetic samples of stannite and kesterite. The interpretation
of their parameters (g- and T-tensors) was refined by computer simulation. The main feature of all the spectra is the unstructured signal centered at about
0.310 T due to the presence of Cu(II). The absence of structure in the signal is due to spin-spin exchange interaction between
Cu(II) and Fe(II), pointing to a diluted distribution of Cu(II). The temperature dependence of the Cu(II) signal can be related
to a topological variation of the first-neighbors coordination. The SQUID measurements, while allowing a more precise interpretation
of the EPR data, led to a full characterization of magnetic behavior of stannite and kesterite down to liquid helium temperature,
evidencing antiferromagnetic interactions between the Fe(II) ions in all samples but in synthetic kesterite. From the EPR
and SQUID experimental data no evidence was provided for the existence of two different structures for stannite and kesterite.
Received: 2 August 1999 / Accepted: 7 January 2000 相似文献
372.
Marco Maffione Stefano Pucci Leonardo Sagnotti Fabio Speranza 《International Journal of Earth Sciences》2012,101(3):849-861
Anisotropy of magnetic susceptibility (AMS) represents a valuable proxy able to detect subtle strain effects in very weakly
deformed sediments. In compressive tectonic settings, the magnetic lineation is commonly parallel to fold axes, thrust faults,
and local bedding strike, while in extensional regimes, it is perpendicular to normal faults and parallel to bedding dip directions.
The Altotiberina Fault (ATF) in the northern Apennines (Italy) is a Plio-Quaternary NNW–SSE low-angle normal fault; the sedimentary
basin (Tiber basin) at its hanging-wall is infilled with a syn-tectonic, sandy-clayey continental succession. We measured
the AMS of apparently undeformed sandy clays sampled at 12 sites within the Tiber basin. The anisotropy parameters suggest
that a primary sedimentary fabric has been overprinted by an incipient tectonic fabric. The magnetic lineation is well developed
at all sites, and at the sites from the western sector of the basin it is oriented sub-perpendicular to the trend of the ATF,
suggesting that it may be related to extensional strain. Conversely, the magnetic lineation of the sites from the eastern
sector has a prevailing N–S direction. The occurrence of triaxial to prolate AMS ellipsoids and sub-horizontal magnetic lineations
suggests that a maximum horizontal shortening along an E–W direction occurred at these sites. The presence of compressive
AMS features at the hanging-wall of the ATF can be explained by the presence of gently N–S-trending local folds (hardly visible
in the field) formed by either passive accommodation above an undulated fault plane, or rollover mechanism along antithetic
faults. The long-lasting debate on the extensional versus compressive Plio-Quaternary tectonics of the Apennines orogenic
belt should now be revised taking into account the importance of compressive structures related to local effects. 相似文献
373.
The NCEP twentieth century reanalyis and a 500-year control simulation with the IPSL-CM5 climate model are used to assess the influence of ocean-atmosphere coupling in the North Atlantic region at seasonal to decadal time scales. At the seasonal scale, the air-sea interaction patterns are similar in the model and observations. In both, a statistically significant summer sea surface temperature (SST) anomaly with a horseshoe shape leads an atmospheric signal that resembles the North Atlantic Oscillation (NAO) during the winter. The air-sea interactions in the model thus seem realistic, although the amplitude of the atmospheric signal is half that observed, and it is detected throughout the cold season, while it is significant only in late fall and early winter in the observations. In both model and observations, the North Atlantic horseshoe SST anomaly pattern is in part generated by the spring and summer internal atmospheric variability. In the model, the influence of the ocean dynamics can be assessed and is found to contribute to the SST anomaly, in particular at the decadal scale. Indeed, the North Atlantic SST anomalies that follow an intensification of the Atlantic meridional overturning circulation (AMOC) by about 9 years, or an intensification of a clockwise intergyre gyre in the Atlantic Ocean by 6 years, resemble the horseshoe pattern, and are also similar to the model Atlantic Multidecadal Oscillation (AMO). As the AMOC is shown to have a significant impact on the winter NAO, most strongly when it leads by 9 years, the decadal interactions in the model are consistent with the seasonal analysis. In the observations, there is also a strong correlation between the AMO and the SST horseshoe pattern that influences the NAO. The analogy with the coupled model suggests that the natural variability of the AMOC and the gyre circulation might influence the climate of the North Atlantic region at the decadal scale. 相似文献