INTERMEDIATE VALENCY, VERWEY TRANSITION AND MAGNETIC-STRUCTURES OF A NEW MINERAL, CU1-EPSILON-FE3-S4, RESULTING FROM THE AGING OF ISOCUBANITE(EPSILON)

A sample of isocubanite (CuFe2S3) collected in a suboceanic hydrotherm al deposit was found to have transformed after a few months into a ver y intimate mixture of similar amounts of chalcopyrite (CuFeS2) and a n ew phase of composition close to CuFe3S4. This new phase is not a diso rdered solid solution like isocubanite, but has a tetragonal superstru cture. Its magnetization, electric resistivity, Fe-57 Mossbauer spectr a, X-ray and neutron diffraction spectra, have been studied at various temperatures. The Fe occupies two different crystallographic sites wi th, at room temperature, a common mixed valency (nominally 2.33+). Thr ee transitions are observed on cooling: at T(N) congruent-to 285 K whe re one of the Fe sites becomes antiferromagnetic; then at T(V) congrue nt-to 190 K, where the second Fe site becomes ferromagnetic with mediu m-range ordering; finally at T(V) congruent-to 105 K, where a first-or der Verwey transition results in the appearance of integral valencies Fe2+ on one site and Fe3+ on the other, and in a rearrangement of the magnetic ordering to a ferrimagnetic structure.