57Fe M�ssbauer study of pumpellyiteokhotskite-julgoldite series minerals

Abstract

Fe and Mn distribution in the pumpellyite group minerals (W 8 X 4 Y 8 Z 12056-n (OH) n ) has been studied by using57Fe Mossbauer spectroscopy. The studied Fe-pumpellyites, belonging to the pumpellyite-julgoldite series, were collected from two localities; metabasites in the Tokoro belt, Hokkaido, and gabbroic sills in the Shimane Peninsula, Japan. Okhotskite, an Mn3+-dominant pumpellyite group mineral, was separated from the ores of metamorphosed manganiferous iron ore deposits in the Tokoro belt.57Fe Mossbauer spectrum of Tokoro Fe-pumpellyite is composed of two Fe2+- and two Fe3+-doublets. On the basis of the single crystal structure refinements of Al-pumpellyites published so far, doublets were assigned to Fe 2+ (IS= 1.01 andQS = 2.73 mm/s), Fe 2+ (IS = 0.97 andQS = 3.18 mm/s), Fe 3+ (IS = 0.29 andQS =1.37 mm/s) and Fe 3+ (IS = 0.36 andQS = 2.09 mm/s), whereIS is isomer shift relative to a metallic iron absorber andQS is quadrupole splitting. The Mossbauer spectrum of the Mitsu Fepumpellyite is composed of three doublets assigned to Fe 2+ (IS= 1.14 andQS = 3.20 mm/s), Fe 3+ (IS = 0.36 andQS =1.13 mm/s) and Fe 3+ (IS = 0.37 andQS= 1.93 mm/s). These assignments show strong preference of Fe3+ in the X-site. The Mossbauer spectrum of the okhotskite is composed of two doublets by Fe 3+ (IS= 0.37 andQS = 1.13 mm/s) and Fe 3+ (IS = 0.42 andQS = 2.18 mm/s). The area ratio shows that Fe 3+ :Fe 3+ ratio is 94:6. On the basis of chemical and Mossbauer analyses, Mn 3+ :Mn 3+ ratio is given as 19:81, indicating stronger Y-site preference of Mn3+ than Fe3+, what is consistent with Jahn-Teller theory. Al, Mn3+ and Fe3+ prefer the Y-site in this order.