Crystal chemistry of two coexisting K-richterites from St. Marcel (Val d'Aosta, Italy)

Two coexisting F-bearing K-richterites from the blueschist-facies metamorphic St Marcel Mn ore deposit have been studied by combined microprobe and IR analytical techniques. Despite their different colours (pink and blue) and cell dimensions, these amphiboles have formulae summing up to nearly 16 cations pfu on the basis of 22 O + 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. Site assignments based on probe and IR compare well with those determined by crystal-structure refinement. However, they also allow detection of empty A sites in the structure. -J.A.Z.