Mössbauer Studies of Some Magnetic Clinopyroxenes

Abstract

The interpretations of the magnetic Mossbauer spectra (MMS) of five synthetic, Fe-rich P21/c clinopyroxenes along the ferrosilite-enstatite (Fs-En) join, and of one species of each of the natural C2/c clinopyroxenes hedenbergite and aegirine are presented. The spectra have been fitted using the HIH for calculating the ferrous contributions to the total line shapes. The enstatite-ferrosilite spectra recorded at temperatures between 0.3 K and 4.2 K are adequately described taking into account a discrete number of components corresponding to the possible next-nearest-neighbour (NNN) configurations of both the M1 and M2 sites. In agreement with literature data it is found that M2-site hyperfine fields are considerably higher than for M1. For both sites the strengths of the hyperfine field depend on the NNN configuration, however in a reversed fashion in the sense that Mg-for-Fe substitution in the M1 sites causes the Fe2+ (M1) fields to increase, and the Fe2+ (M1) fields to decrease. The M2 hyperfine fields decrease with increasing Mg content, while for M1 no clear tendency is observed. The spectra of the hedenbergite sample at temperatures between 4.2 K and the Neel temperature of 33 K are best described by a model-independent hyperfine-field distribution (HFD) especially those at the higher temperatures. The distribution at low temperatures is narrow and close to symmetric. The average hyperfine field at 4.2 K (180 kOe) is higher than the fields acting at the M1 sites in the Fs-En samples (83 kOe for the ferrosilite end-member) and possible reasons for the field enhancement are discussed. Some of the derived Mossbauer parameters disagree with reported values, which is demonstrated to be due to different fitting procedures. The temperature variation of the hyperfine field suggests a rectangular 2D Ising-type behaviour. The line shape of the 4.2 K spectrum of the aegirine sample is characteristic for a broad hyperfine-field distribution for the Fe3+ cations at M1 sites. Fitting the spectrum with a superposition of a ferrous and a ferric HFD revealed three major Fe3+ contributions with fields of 287, 376, and 468 kOe, respectively. These fields are low because the relatively high value of the reduced temperature, i.e., T/TN=0.4. The presence of these three distinct components is in disagreement with the NNN models.