JOHNINNESITE, NA-2(MN2- A NEW OCCURRENCE IN VAL-FERRERA (GRAUBUNDEN, ZERLAND)()(9)(MG,MN)(7)(ASO4)(2)(SI6O17)(2)(OH)(8) )

The rare mineral johninnesite, a-2(Mn2+)(9)(Mg,Mn)(7)(AsO4)(2)(Si6O17) (2)(OH)(8), has been discovered within a metamorphic Fe Mn-deposit in Triassic marbles of the Schams nappes in Val Ferrera (Graubunden, Swit zerland). To our knowledge, this is the second occurrence of this mine ral,which was first described from Fe-Mn-ore lenses at the Kombat mine (Namibia). Johninnesite from Val Ferrera differs from the type materi al by its Fe and V contents, and by higher MgO contents which are corr elated to lower MnO contents. IR and Raman spectra for this species ar e reported for the first time. Johninnesite is a product of the metamo rphism of syn-genetic exhalative deposits, and the reason for the rari ty can be ascribed to the geochemistry of the protolith. In particular , if the protolith is not depleted in aluminium, the widespread silico -arsenate ardennite will form instead of johninnesite.