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M. Borgheresi F. Di Benedetto A. Caneschi G. Pratesi M. Romanelli L. Sorace 《Physics and Chemistry of Minerals》2007,34(9):609-619
A magnetic and spectroscopic characterisation has been performed on a natural bornite sample from the Natural History Museum
of the University of Florence. The combination of magnetic measurements and continuous-wave electron paramagnetic resonance
(cw-EPR) spectroscopy at different temperatures and frequencies provided information about the distribution and valence states
of Cu and Fe in bornite. The studied sample was found to obey the Curie–Weiss law with a transition from a paramagnetic to
an antiferromagnetic phase at 64 K; its possible attribution to a disordered spin glass phase was ruled out by ac susceptibility
measurements. Q- and X-band cw-EPR measurements confirmed the presence of Fe(III) as fundamental valence state in bornite:
the single EPR line registered in the temperature range from 300 to 65 K can be assigned, in fact, to the Fe(III) single ions.
Some Cu(II) signals were revealed in the low temperature EPR spectra and attributed to an early stage of the surface alteration.
The width of the Fe(III) EPR spectrum, which hinders any characteristic spectral structure, can be ascribed to the exchange
interaction. The pure antiferromagnetic character of the magnetic transition confirms the ordering between Fe and Cu in the
bornite structure, at least at low temperature (≤70 K). Moreover, the relatively high Nèel temperature suggests the accepted
model of Collins et al.’s (Can J Phys 59:535–539, 1981) to conveniently explain the overall magnetic properties in the range 298–4 K. Despite the increasing of the susceptibility
in the paramagnetic range, in fact, the integrated EPR line area decreases by lowering the temperature, thus suggesting a
progressive rising of the antiferromagnetic interactions among next-nearest-neighbouring paramagnetic centres. 相似文献
2.
G. P.?Bernardini M.?Borgheresi C.?Cipriani F.?Di BenedettoEmail author M.?Romanelli 《Physics and Chemistry of Minerals》2004,31(2):80-84
A study on Zn replacement in sphalerite by divalent Mn has been performed by using EPR spectroscopy. Twelve natural and synthetic samples with different Mn-contents were selected for the spectroscopic investigation. The EPR spectra vary from an hyperfine sextet to a single broad line according to their Mn-content. Three components, attributed to Mn(II) in different chemical coordination (namely isolated ions, interacting ions, clusters) have been determined by means of spectral simulation. The variations of the EPR signals among the different spectra are due only to the mixing of these components, without any variation of each spectral parameter. Differing distribution of Mn(II) was, therefore, observed at a local scale. Consequently, the specific interactions driving the coalescence of Mn(II) ions to form clusters may be responsible for the banded pattern in sphalerite crystals. 相似文献
3.
Application of Compositional Techniques in the Field of Crystal Chemistry: A Case Study of Luzonite, a Sn-Bearing Mineral 总被引：1，自引：1，他引：0
Miria Borgheresi Antonella Buccianti Francesco Di Benedetto David John Vaughan 《Mathematical Geosciences》2013,45(2):183-206
Out-of-equilibrium crystallization often produces complex compositional variability in minerals, generating zoning and other mixing phenomena. The appropriate microchemical characterization of the resulting out-of-equilibrium patterns is of critical importance in understanding the overall physical and chemical properties of the host crystalline phases. In this framework, the modeling of compositional changes assumes a fundamental role. However, when compositional data are used, their management with standard exploratory, statistical, graphical, and numerical tools may give misleading results attributable to the phenomenon of induced correlations. To avoid these problems, methods able to extract compositional data from their constrained space (the simplex) in order to apply standard statistics, have to be adopted. As an alternative, the use of tools having properties able to work in the simplex geometry has to be considered. A luzonite single crystal (ideal composition, Cu3AsS4) exhibiting concentric and sector zoning was studied using electron probe microanalysis in order to understand the mechanisms which give rise to chemical variability and conditions in the developing environment. Compositional variations were determined by collecting data along three different transects. The major and minor elements (Cu, As, S, Fe, Sb, Sn) were analyzed with the aim of characterizing their patterns of association in the crystal and, hence, crystal evolution. The whole covariance structure as well as the chemical relationships between the successive zones was investigated by means of compositional methods, considering both data transformation and the stay in the simplex approach. Results indicate that the crystal grew under quiescent conditions, where chemical control was primarily exercised by the mineral’s surface and only minor effects were due to changes in the composition of the surrounding fluid. Consequently, an oscillatory uptake of chemical components occurred in which a competition between famatinite-like (Cu3SbS4) and kuramite-like (Cu3SnS4) domains characterized the As-poor zones. 相似文献
4.
M. Borgheresi G. P. Bernardini C. Cipriani F. Di Benedetto M. Romanelli 《Mineralogy and Petrology》2005,85(1-2):3-18
Summary A minero-chemical and spectroscopic characterisation has been performed on a natural bornite sample from the Natural History
Museum of the University of Florence, through the use of EPMA, XRD, continuous-wave (cw) and pulsed EPR spectroscopy. The
use of different EPR techniques allowed to fully investigate the distribution and valence states of Cu and Fe and to study
the early oxidation products of bornite.
The microanalytical and XRD characterisation proved bornite to be homogeneous and stoichiometric. Cw-EPR measurements confirmed
the presence of Fe(III) as fundamental valence state in bornite; moreover, no evidence of Cu(II) as a bulk transient species,
due to a charge transfer process, has been revealed. The broadening of the Fe(III) EPR spectrum is ascribed to a partial transfer
of spin density from Fe(III) to the surrounding cationic sublattice.
In contrast, the pulsed EPR experiments, registered at 4.2 K, revealed the presence of both Fe(III) and Cu(II). These species
were supposed to be present at the early stage of the surface alteration. The data provided by the electron spin-echo decay
and modulation suggest both the oxidised Cu(II) and Fe(III) species to belong to premature surface water adducts (evidenced
by proton nuclear modulation), which may evolve to sulphate species. 相似文献
5.
F. Di Benedetto G. B. Andreozzi G. P. Bernardini M. Borgheresi A. Caneschi C. Cipriani D. Gatteschi M. Romanelli 《Physics and Chemistry of Minerals》2005,32(5-6):339-348
An extensive study of natural and synthetic Fe-bearing sphalerites (Fe-content ranging from 0.005 afu to 0.250 afu) has been carried out through the combined investigations of the temperature dependence of the magnetic susceptibility and of the 57Fe Mössbauer spectroscopic features. Magnetic susceptibility measurements evidence, in the low temperature regime, the presence of clustered Fe even in the most diluted samples, whereas all the Mössbauer spectra are described in terms of variable proportions of three components, namely one single and two doublets absorption, centred at the same isomer shift, relative to Fe ions and to different Fe clusters, respectively. The comparison of all the experimental results points out a self-affinity of Fe ions in sphalerite favoured by the superexchange interaction, which stabilises the formation of clusters even in diluted samples and may be related to both macroscopic inhomogeneous distribution and crystal zoning. 相似文献
6.
F. Di Benedetto T. Evstigneeva M. Borgheresi A. Caneschi M. Romanelli 《Physics and Chemistry of Minerals》2009,36(6):301-309
The magnetic properties of the synthetic Cu3SnS4 (kuramite)–Cu2FeSnS4 (stannite) pseudobinary series were investigated by means of electron paramagnetic resonance (EPR) spectroscopy, at room
temperature, and by magnetometry, in the range 2–300 K. The system is particularly complex, from both chemical and crystal
chemical points of view, in particular with respect to the metal valence states and the local ordering in the different terms
of the series. Nevertheless, recent successes in synthesising nanostructured kuramite foster the interest to ascertain the
bulk magnetic properties of these important semiconducting phases. The obtained results allowed to ascertain that a variable
lack of local ordering in the Cu3–x
Fe
x
SnS4 (x < 0.85) samples induce the raise of strong metal–sulphur–metal superexchange interactions, that result in the appearance
of marked deviations from the single-ion behaviour, typical for pure stannite. Ferro- and antiferromagnetic interactions are
in fact observed at relatively high temperatures (~150 K). A possible role played by Cu(I)–Fe(III) was revealed by the EPR
measurements. The Cu-rich terms of the series (x < 0.1) are characterised by dynamic resonant disorder (i.e. time-evolving delocalisation of the formally divalent valence
state for Cu among the nearest neighbouring Cu-sites), in addition to the Cu–Fe–Sn static disorder verified along the whole
series. Both factors concurring to a non-periodic arrangement of paramagnetic ions in the lattice have the main effect to
drastically broaden the EPR lines. 相似文献
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