Oxidation state of Fe in olivine in andesitic scoria from Kasayama volcano, Hagi, Yamaguchi Prefecture, Japan

摘要

The oxidation state of an olivine phenocryst separate from clinopyroxene-olivine andesitic black scoria with red-brown tint from Kasayama volcano, Hagi, Yamaguchi Prefecture, was determined using ~(57)Fe Mossbauer spectroscopy, to examine the effect of high temperature oxidation on the oxidation state of Fe. The olivine phenocrysts examined were euhedral to subhedral in form, typically fractured, and about 0.2-1 mm in maximum dimension. Small amounts of Fe-bearing precipitate minerals often occur on the rims of olivine phenocrysts in the black scoria with red-brown tint, and near subhedral Cr-rich magnetite with about 5 um in diameter. Olivine phenocrysts lacking such precipitates and inclusions were separated from a sample using an isodynamic separator and handpicking under a binocular microscope. Examination by optical microscopy, electron microprobe analysis, Raman spectroscopy, and high-resolution transmission microscopy proved no precipitates in the separated olivine phenocrysts. Average Fo value and Fe content of the olivines were 81 mol% and 0.36 atoms per formula unit (apfu), respectively. The ~(57)Fe Mossbauer spectrum of the olivine separate consisted of three doublets assigned to Fe~(2+) at the Ml, Fe~(2+) at the M2 and Fe~(3+) at the octahedral sites. The Fe~(2+):Fe~(3+)-ratio is 95(3):5(1). By applying this value to the average Fe content, Fe~(2+) and Fe~(3+) are calculated to be 0.34(1) and 0.018(4) apfu, respectively. Since the Fe~(3+) within olivine detected in this study is not due to any Fe~(3+)-bear-ing impurity, Fe~(3+) could be located at the octahedral site within the olivine structure. The quadrupole splitting value of the Fe~(3+)-Moessbauer doublet [0.53(5) mm/s] in Kasayama olivine is significantly lower than published data for Fe~(3+) at the M2 site in olivine, and in laihunite. This suggests possible distribution of Fe~(3+) at the Ml site. Fe~(3+) within olivine in the black scoria with red-brown tint is considered to have been generated at high temperatures (perhaps above 800 ℃).