⁵⁷Fe Mössbauer spectra of natural and synthetic hastingsites, and implications for peak assignments in calcic amphiboles

In order to test the validity of unique assignments of ⁵⁷Fe Mössbauer spectral peaks to individual crystallographic sites in hornblendes, spectra were collected for six synthetic hastingsites, ideally $NaCa2Fe42+Fe3+Si6Al2O22(OH)2$⁠, and for three natural hornblendes of similar compositions. The synthetic, end-member hastingsites are inferred from compositional constraints to have the M(1), M(2), and M(3) sites filled entirely with iron and to have little or no iron in the M(4) site. Thus, these samples provide a test of peak-to-site correspondence. The areas of the Fe²⁺ doublets in the synthetic samples are not, however, in the same proportions as the abundances of the sites to which they are conventionally assigned, as would be expected if each doublet uniquely represented the contribution from a single site. On the other hand, the fractional areas of the three Fe²⁺ doublets, relative to the total Fe²⁺ absorption normalized to unity, show relatively uniform values of 0.55±0.03, 0.29±0.03, and 0.16±0.02 for the outer, intermediate, and inner doublets, respectively, in the synthetic and natural hastingsites from this study, as well as for other hornblendes from the literature exhibiting a range of total iron and Fe³⁺ contents. By inference from their chemical analyses, the natural samples are also considered to contain little or no iron in M(4). These results suggest that the ferrous absorption has a characteristic shape independent of iron concentration and that the ferrous doublets cannot be assigned to sites uniquely to yield accurate site populations for hornblendes. The observed ferrous area fractions cannot be explained by peak broadening due to octahedral next-nearest-neighbor effects, although it is possible that the presence of neighboring tetrahedral aluminum may be a dominant factor in perturbing the areas of the ferrous doublets.