Abstract

The Mössbauer spectra of calcic amphiboles have been reassessed to examine the validity of previous peak assignments. A brief summary of previously-established peak assignments for Mg-Fe, calcic and sodic amphiboles is presented. The Mössbauer spectra of tremolite-actinolites are reinterpreted in terms of Fe2+ in the M(1) and M(3) sites (quadrupole splitting, Δ ∼ 2.8 mm/sec), the M(2) site (Δ ∼ 2.2 mm/sec) and the M(4) site (Δ ∼ 1.8 mm/sec). From a number of spectra, these assignments indicate that Fe2+ prefers to enter the M(4) site prior to the M(1) and M(3) sites, and it discriminates against the M(2) site. This is the same ordering scheme found in cummingtonite-grunerites. Increasing the Al and Fe3+ contents in calcic amphiboles reduces Δ for Fe2+ in the M(1), M(2) and M(3) sites from the values in tremolite-actinolites, and separates the M(l) and M(3) doublets. It is suggested that next-nearest-neighbor effects on the already closely-overlapping peaks limit the accuracy of determining site populations from Mössbauer spectra of calcic amphiboles.

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