Abstract

Two coexisting F-bearing K-richterites from the blueschist-facies metamorphic manganese ore deposit St. Marcel (Val d’Aosta, Italy) have been studied by combined empa and ir analytical techniques. Despite their different colors (pink and blue) and cell dimensions, these amphiboles have formulae summing up to near 16 cations pfu on the basis of 220 + 2(OH + F) and are chemically almost identical. The major differences are in the F/OH ratio and the Ca content, and significant minor differences occur in Fe and Ti.

The ir spectra in the OH-stretching region are notably different, probably as a result of the different F contents. However, both amphiboles have a prominent absorption peak at 3731 cm-1, typical of the K-OH-(MgMgMg) configuration, plus weak bands at 3672 and 3653 cm-1, typical of configurations having the A site empty. Moreover, pink K-richterite has a strong absorption band at 3715 cm-1 that was assigned to the Sr-OH-(MgMgMg) configuration, balancing the A sites that are empty. Site assignments based on empa and ir compare well with those determined by crystal-structure refinement. However, they also allow detection of empty A sites in the structure.

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