Project 2 Project Research on Advances in Isotope-Specific Studies Using Muti-Element Mössbauer Spectroscopy (R2P2)

概要

INTRODUCTION: Cordierite ((Mg,Fe)2Si5Al4O18) is a rock-forming mineral which belongs to cyclosilicate. The crystal structure of cordierite is characterized by honey- comb rings. The honeycomb rings of cordierite consist of four SiO4 and two AlO4 tetrahedra. Governed by the Al- avoidance rule, two Al ions occupy opposite sites in the honeycomb rings, resultantly, Si and Al ions are ordered in tetrahedral site. Ordered cordierite belongs to the ortho- rhombic crystal system. In cordierite which crystallized at high temperature, Si and Al ions are arranged in disorder state. As the result, the disordered cordierite belongs to hexagonal crystal system. In the crystal structure of cordi- erite, Fe ion occupies a octahedral site which is surrounded by three tetrahedral sites including Si and Al. Five Si and four Al occupy the added nine tetrahedral site in the unit cell of cordierite. Since nine Si and Al are arranged in the ordered state in the orthorhombic cordierite, Si and Al around the octahedral site are also ordered. On the other hand, Si and Al around the octahedral site disordered in the hexagonal cordierite. Mössbauer spectra of cordierite show quadrupole doublet.
　Intensities of component peaks in a quadrupole doublet of a thin section as a single crystal are asymmetric and vary depending on the angle between the direction of incident -rays and the crystallographic orientation of the thin sec-tion. Intensity of quadrupole doublet (Ih / Itotal ) means a ratio between area of the peak of the higher energy (Ih) and total area of the doublet (Itotal= Ih + Il) (sum of Ih and area of the lower energy (Il)). The intensity of component peaks of a 57Fe Mössbauer doublet is related to an electronic field gradient (EFG) tensor of the site containing Fe2+ (Zimmer- mann, 1975 and 1983). Electric gradient is in inverse pro- portion to the cube of distance between the Mössbauer nu- clei and the neighboring atoms. How far is electric gradi- ent at Mössbauer nuclei influenced by the neighboring at- oms?
　In hexagonal and orthorhombic cordierite, the first neighboring atoms are commonly oxygens. However, the second neighboring atoms in the orthorhombic cordierite are expected to be different from those of hexagonal cor- dierite. To compare quadrupole doublet of cordierite, Mössbauer spectra of hexagonal and orthorhombic cordi- erite were measured. In this study, single crystals of natu- ral cordierite were used for this study. In the crystal, black and white crystals coexist. The black crystal shows hexagonal X-ray diffraction pattern, and the white crystal shows orthorhombic X-ray diffraction pattern. Crystallo- graphically oriented thin sections perpendicular to c* was prepared by measuring X-ray diffraction patterns by using X-ray precession camera. Mössbauer spectra of thin sec- tion of single crystal of cordierite were measured under - ray parallel to the c*-axis.

EXPERIMENTS and RESULTS: Mössbauer measure- ments were carried out in transmission mode on a constant acceleration spectrometer with an Si-PIN semiconductor detector (XR-100CR, AMPTEK Inc.) and multi-channel analyzer of 1024 channels. A 3.7GBq 57Co/Rh of 4mm in diameter was used as -ray source. An 57Fe-enriched iron foil was used as velocity calibrant. The two symmetric spectra were folded and velocity range was ±5mm/s. Thickness corrections of raw spectra were not done.
　Fig.1. Mössbauer spectrum of hexagonal cordierite meas- ured under -ray parallel to the c*-axis. Isomer shift, Q- splitting, line width and intensity of quadruploe doublet were 1.21, 2.30, 0.32 mm/s and 0.31, respectively. Ortho- rhombic cordierite shows quite resemble spectrum to the hexagonal cordierite The Mössbauer parameters were also the same values. The results suggest that the second neigh- boring atoms give little effect to electric gradient at Mössa- bauer nuclei.

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