Gian Piero Bernardini

Compositional zoning in sphalerite crystals

Abstract series of natural sphalerite samples, characterized by an inhomogeneous (zonal) distribution of Fe, Mn, and Cd atoms substituting for Zn, has been investigated by electron probe microanalysis and X-ray element distribution mapping. The cation distributions are markedly inhomogeneous due to the reciprocal effects of the minor elements on their respective solubilities. In particular, zonal partitioning between Fe and Mn and between both cations and Hg was observed in sphalerite, as well as coupled Cu-In and Cu-Fe substitutions. Distinctly different distribution patterns were identified in Mn-free samples with a low Fe-content, as compared to Mn-bearing samples. In the former case, Fe and Cd distribution patterns are very similar, whereas in the latter case, Cd is distributed homogeneously and Mn and Fe patterns are antithetic. The different oscillatory zoning observed in Mn-free samples is attributed to a fast, self-organized solute (Fe, Cd) deposition, although an alternative external origin cannot be ruled out. On the other hand, in the presence of Mn, zoning may be related to an absorption process at the mineral-fluid interface controlled by a competition between Mn and Fe that may explain the observed limited coupled concentration of these elements in sphalerite. The homogeneous distribution of Cd suggests relatively slow crystal growth

Chemical speciation of Ag in galena by EPR spectroscopy

Abstract Electron paramagnetic resonance (EPR) spectroscopy has been used to study the valence state of silver in “argentiferous” galena samples from the Apuane Alps (Tuscany, Italy) mining district. This method was used to reveal primary metallic silver (Ag0) in galena. Both thermodynamic data and experimental studies suggest that galena and native silver can stably coexist, but have not been reported as a primary (hypogene) assemblage in natural samples. EPR spectroscopy proved to be a suitable tool to solve this problem, because this technique is capable of detecting paramagnetic species down to the ppb level, even in a highly absorbent matrix such as galena. A detailed SEM-EDS investigation could not detect metallic silver (or gold) in galena samples, but did reveal small (few micrometers) inclusions of Ag-bearing phases, in which silver has a formal valence of +1. On the other hand, EPR spectra indicated the presence in galena of pairs and clusters of elemental silver atoms, which may be associated with pairs of metallic gold, or with silver-gold hetero-atomic pairs. Therefore, SEM/EDS and EPR are complementary techniques, revealing the presence of both Ag+ and Ag0. The Ag(Au) metallic species were apparently deposited on the galena surface during its growth from mineralizing fluids. Their scarcity, and the presence of larger amounts of Ag1+ phases, suggest that the assemblage galena-metallic silver was stable only under peculiar physical and chemical conditions. The formation of Ag0 was presumably linked to local and rare chemical fluctuations of the hydrothermal environment, characterized by low activities of S, Sb, Bi (Cu…) and high activity of Ag in the fluids. The occurrence of both homo- and hetero-atomic pairs suggests either different kinetics of pair formation, or possible fluctuations in the composition of the hydrothermal fluids, which alternatively carried Ag or Au species, or both

57Fe Mössbauer and electronic spectroscopy study on a new synthetic hercynite-based pigment

AbstractA new blue hercynite-based pigment,stable at the processing temperatures for the ceramic application,has been synthesized. It isconstituted by corundum and Ti-bearing hercynite. In order to clarify the complex Fe distribution evidenced by Baldi et al. [Proc.CIMTEC 2002(2003)],a 57 Fe Mo¨ssbauer and diffuse reflectance spectroscopy study has been undertaken. Fe was found to be dis-tributed between corundum,as trivalent,and hercynite,both as Fe(II) and Fe(III). Moreover,Fe(II) occupies both tetrahedral andoctahedral sites in the latter mineral,according to a temperature dependent disorder. UV–VIS investigation revealed the presence ofa broad band in the blue region,the shape and position of which was found to be due to an intervalence charge transfer mechanismbetween Fe(II) and Ti(IV) in two edge-sharing octahedra of hercynite.# 2003 Elsevier Ltd. All rights reserved. Keywords:Al 2 O 3 ; Colour; Pigment; Spectroscopy; Spinels 1. IntroductionCeramic pigments are white or coloured crystallinematerials with high thermal stability and high chemicalresistance. These properties are strategic for their use inhigh temperature treatments. In recent years the devel-opment of new ceramic materials has fostered theresearch of pigments stable at application temperaturesover 1200 C. The crystalline coloured substances thatremain unaltered in such conditions belong to a restric-ted number of structures: zirconium silicate ZrSiO

EPR and magnetic investigations on sulphides and sulphosalts

Magnetometric studies of sulphide and sulphosalt minerals were carried out using electron paramagnetic resonance (EPR) and a superconducting quantum interference device (SQUID). These have proved to be powerful tools to investigate not only the valence states of transition elements (TE) cations but also the site symmetries and magnetic interactions between neighbouring atoms. While only few investigations are found in literature concerning sulphide minerals in recent years, advanced resonance techniques, such as electron spin echoes (ESE) and high-frequency EPR (HF 2 EPR), allow us to supplement and extend the information that can be obtained with standard techniques. Sphalerite derivative structures, such as stannite and tetrahedrite, were object of a systematic and advanced EPR and SQUID investigation. Synthetic analogues with simple chemical composition are here used as reference materials for the interpretation of the spectra refined by computer simulation. Spectroscopic investigations on these materials allow us to establish the different nature of “free” electrons delocalized with respect to the usual unpaired electrons of Cu(II), while preliminary studies on the ZnS-archetype, doped with TE, prove to be very useful in refining the site symmetries of cations in the structure.

Estimation of rate coefficients from the overall amount of material reacted at various scan rates

A method to evaluate the temperature dependence of the reaction rate from the variation of the amount reacted at the end of a number of experiments at various scan rates is deduced from the fundamental equations of DTA. The method was applied to the synthesis of Paakkonenite and gave parameters in good agreement with those determined for the same reaction by using other experimental data and the differential method of Sharp and Wentworth. Furthermore, an empirical procedure previously used to get the same results is compared with the present elaboration. The parameters evaluated are reported and discussed.

Crystal chemistry and magnetic properties of Cu5FeS4(bornite): a Mössbauer study

24 European Crystallographic Meeting, ECM24, Marrakech, 2007 Page s265 Acta Cryst. (2007). A63, s265 MS41 P01 Crystal chemistry and magnetic properties of Cu5FeS4 (bornite): a Mossbauer study. M. Borgheresi, F. Di Benedetto, M. Romanelli, G.P. Bernardini, W. Lottermoser, G. Tippelt, G. Amthauer, Department of Earth Sciences and Natural History Museum, University of Florence, Italy. Department of Chemistry, University of Florence, Italy. Department of Materials Engineering and Physics, University of Salzburg, Austria. E-mail: georg.amthauer@sbg.ac.at