57Fe Mossbauer study of stilpnomelane and associated chlorite from polish granite pegmatites

Abstract

This paper reports the results of 57Fe Mössbauer study of stilpnomelane K0.6Fe6(Si8Al)(O,OH)27 .2H2O from Żółkiewka (Lower Silesia, Poland). Stilpnomelane in paragenesis with chlorite, tourmaline, claevelandite and zeolites has been found in granite pegmatites in quarries at Żółkiewka. The samples of naturally weathered stilpnomelane and chlorite from the same fragment of rock were also studied. The Mössbauer spectrum of an untreated sample could be fitted to two Fe2+ doublets and two Fe3+ doublets. The Fe2+ 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 “fingerprint” of this mineral. After heating at 200°C, the relative content of Fe2+, ΣFe2+/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 Mössbauer spectrum consists of two sextets and two ferric doublets. Abnormally high content of Fe2+ 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 Fe2+ and Fe3+ doublets in weathered stilpnomelane and associated chlorite suggest that the process of chlorite transformation into stilpnomelane seems to be very probable.