57Fe Mossbauer-effect studies of Ca-rich, Fe-bearing clinopyroxenes: Part I. Paramagnetic spectra of magnesian hedenbergite

摘要

Mössbauer spectra (MS) of two natural magnesian hedenbergite samples (hereafter HED1 and HED2) were collected at temperatures in the range 35 to 800 K. At selected temperatures a longitudinal external field of 60 kOe was applied to the absorbers. The samples were observed to order magnetically at T_N = 33 ± 1 K and 27 ± 1 K, respectively. The temperature dependencies of the Fe2+ center shifts, , were analyzed using the Debye model for the lattice vibrations, including a temperature variation for the intrinsic isomer shift, _I. The characteristic Mössbauer temperatures, _M, of HED1 and HED2 were found to be 440 ± 20 K and 490 ± 20 K, and the intrinsic isomer shifts, _I, to be 1.434 ± 0.005 mm/s and 1.440 ± 0.005 mm/s, respectively, with a linear correlation coefficient, a, between _I and T of –4.5 x 10–5 mm/sK. From the external-field (60 kOe) MS recorded at 83, 147, 223, and 277 K for HED2, the principal component of the electric field gradient (EFG), V_zz, is determined to be positive and the asymmetry parameter 0.70 0.80. Considering the discrepancy between the calculated and experimental applied-field MS, the obtained results for HED1 are assumed to be somewhat less accurate. The temperature variations of the quadrupole splitting, E_Q(T), have been interpreted using the crystal-field model. Two approaches have been applied to evaluate the crystal field. In both cases the crystal-field Hamiltonian included the spin-orbit coupling. The first model emanates from the approximate and simplified symmetry of the ferrous sites, whereas the second takes into account the real symmetry of the sites, thus leading to a point-charge calculation. The temperature variations E_Q(T) and (T) could be successfully described using the latter approach.