Structural variation of babingtonite depending on cation distribution at the octahedral sites

摘要

Babingtonite, Ca_2Fe~(2+)Fe~(3+)[Si_5O_(14)(OH)] (Z=2, space group P1) from Yakuki mine (Japan), Gronsjoberget (Sweden), Kandivali Quarry (India), Baveno Quarry (Italy), Brastad Mine (Norway), and Kouragahana (Japan), and manganbabingtonite, Ca_2(Mn~(2+), Fe~(2+))Fe~(3+)[Si_5O_(14)(0H)], from Iron Cap mine (USA) were studied using electron-microprobe analysis (EMPA), ~(57)Fe Mossbauer analysis and single-crystal X-ray diffraction methods to determine the cation distribution at Ml and M2 and to analyze its effect on the crystal structure of babingtonite. Although all studied babingtonite crystals are relatively homogeneous, chemical zonation due to mainly Fe <-> Mn substitution is observed in manganbabingtonite. Mossbauer spectra consist of two doublets with isomer shift (I.S.)= 1.16-1.22 mm/s and quadru-pole splitting (Q.S.)=2.33-2.50 mm/s and with 7.5. =0.38-0.42 mm/s and Q.S. =0.82-0.90 mm/s, assigned to Fe~(2+) and Fe~(3+) at the Ml and M2 octahedral sites, respectively.