57Fe Mossbauer study of the oxidation state of iron in stillnomelane from granite pegmatites in Poland

The effects of heat treatment on stilpnomelane from granite pegmatites at Zolkiewka (Lower Silesia, Poland) were studied using Fe-57 Mossbauer spectroscopy. Samples of naturally altered stilpnomelane and chlorite were also studied. The Mossbauer spectrum of an untreated sample was fitted to two Fe2+ doublets and two Fe3+ doublets. The Fe2+ doublets had similar isomer shifts (1.13 and 1.16 mm/s for the octahedral M I and M2 sites), different quadrupole splitting values (QS = 2.28 and 2.66 mm/s, respectively), and an M I:M2 area ratio of nearly 1:3. For ferric iron, the first doublet was assigned to the combined M1 + M2 + M4 octahedral positions and had IS = 0.41 and QS = 1.18 mm/s. The second doublet was assigned to the M3 site, with parameters IS = 0.36 and QS = 2.07 mm/s. This is characteristic for stilpnomelane. The oxidation of Fe2+ is fast at low temperatures, with a maximum at 360 degrees C. Near 1050 degrees C, the stilpnomelane structure broke down completely, and the spectrum consisted of two sextets and two ferric iron doublets. The predominant phase in this new material is hematite, as represented by the two sextets. The abnormally high fractional Fe2+ content in the altered stilpnomelane (0.70) as compared to the untreated sample (0.45) indicates that the sample was subjected to local hydrothermal processes. Nearly identical divalent iron contents and hyperfine parameters for the Fe 21 and Fe3+ doublets (except FC3+ in M3) in the altered stilpnomelane and associated chlorite strongly Suggest that chlorite is transformed into stilpnomelane.