Magnetic properties, self-reversal remanence and thermal alteration products of smythite

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This study is a follow up of the investigation of some magnetic properties and metastability of greigite in samples obtained from Miocene claystones in the Krusne hory (Erzgebirge) Piedmont basins (Bohemia). Three different methods of upgrading the smythite were applied; the magnetic properties of the concentrates are compared. The thermal conversion of smythite sets in at 200°C while greigite converts at 250°C. The first intermediate products to be formed are iron sulphides, marcasite clearly dominating over pyrite and pyrrhotite. Apart from a Fe3+ sulphate with a composition of Fe2(SO4)3, oxidation of these sulphides gives rise to α-Fe2O3. The result of the subsequent decomposition of the mentioned sulphate is the formation of η-Fe2O3, which retains the sulphate structure. The final product of the thermal decomposition at 800°C is α-Fe2O3. In the smythite concentrate the conversion to Fe3+ sulphate and η-Fe2O3 is about twice as intensive as in greigite. No direct conversion to γ-Fe2O3 was found. During the thermal process self-reversals of remanence were observed, in various samples as many as four reversals in the temperature interval from 340 to 590°C. The occurrences of self-reversals of remanence were only observed at high degrees of thermal demagnetization, of the order of 10−2 down to 10−3 in the temperature interval of sulphide origin (below 400°C), and of the order of 10−4 down to 10−6 in the temperature interval of Fe-oxides origin (above 400°C).