57 Fe Mössbauer study of stilpnomelane and associated chlorite from Polish granite pegmatites

This paper reports the results of 57 Fe Mossbauer study of stilpnomelane K0.6Fe6(Si8Al)(O,OH)27 . 2H2O from Żolkiewka (Lower Silesia, Poland). Stilpnomelane in paragenesis with chlorite, tourmaline, claevelandite and zeolites has been found in granite pegmatites in quarries at Żolkiewka. The samples of naturally weathered stilpnomelane and chlorite from the same fragment of rock were also studied. The Mossbauer spectrum of an untreated sample could be fitted to two Fe 2+ doublets and two Fe 3+ doublets. The Fe 2+ doublets have similar isomer shifts, but they distinctly differ in values of quadrupole splittings. Ferric iron, octahedrally coordinated occurs in three different environments represented by two doublets. The first of them, assigned to combined M1 + M2 positions and the second one assigned to M3 site with parameters IS = 0.36 mm/s and QS = 2.07 mm/s is visible in the spectra up to 880°C. This doublet is characteristic of stilpnomelane and can be regarded as a "finger- print" of this mineral. After heating at 200°C, the relative content of Fe 2+ , ΣFe 2+ /ΣFe, decreases to 0.22 what means a 50% decrease in comparison to the initial value observed in the untreated sample. At about 1000°C, complete breakdown of the stilpnomelane structure takes place and the Mossbauer spectrum consists of two sextets and two ferric doublets. Abnormally high content of Fe 2+ in naturally weathered stilpnomelane (0.70) comparing to the untreated sample of stilpnomelane (0.45) indicates that the sample had to be subjected to hydrothermal processes operating locally. Almost the same contents of divalent iron and hyperfine parameters of Fe 2+ and Fe 3+ doublets in weathered stilpnomelane and associated chlorite suggest that the