CRYSTAL CHEMISTRY OF TETRAHEDRITE SOLID-SOLUTION: EPR AND MAGNETIC INVESTIGATIONS

Abstract

One hundred and thirty samples from the collection of the Natural History Museum in Florence, labeled as tetrahedrite, have
\nbeen extensively studied by means of SEM, XRD, EPMA, DTA, EPR and SQUID techniques. Wide-ranging substitutions,
\nconfirmed by compositional data, play a fundamental role in the thermal behavior, “stabilizing” natural tetrahedrite with respect
\nto the synthetic equivalent. In order to determine the valence state and site occupancies of Cu and Fe, electron paramagnetic
\nresonance (EPR) and magnetic measurements were performed on selected samples of natural tetrahedrite. EPR measurements
\nwere performed down to 130 K, and magnetic susceptibility measurements, in the range 2–300 K. EPR magnetic parameters were
\ndetermined on the basis of spectral simulations. The behaviour of ––1 versus temperature was interpreted by means of
\nHeisenberg’s model, thus yielding values for the Curie and Weiss constants. All samples are characterized by the presence of
\nsmall amounts of Cu2+ and Fe2+; Fe3+ was detected only in metal-deficient samples. Both Cu and Fe occupy the tetrahedral site;
\nwhereas in some samples the former appears aggregated in dimers, the latter is randomly distributed over the lattice. The results
\nof the study confirm the crystal-chemical formula of the tetrahedrite samples investigated.