The rare mineral johninnesite, Na2(Mn2+)9(Mg,Mn)7(AsO 4)2(Si6O17)2(OH) 8, has been discovered within a metamorphic Fe-Mn-deposit in Triassic marbles of the Schams nappes in Val Ferrera (Graubünden, Switzerland). To our knowledge, this is the second occurrence of this mineral, which was first described from Fe-Mn-ore lenses at the Kombat mine (Namibia). Johninnesite from Val Ferrera differs from the type material by its Fe and V contents, and by higher MgO contents which are correlated to lower MnO contents. IR and Raman spectra for this species are reported for the first time. Johninnesite is a product of the metamorphism of syn-genetic exhalative deposits, and the reason for the rarity 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.
|Number of pages||7|
|Journal||Schweizerische Mineralogische und Petrographische Mitteilungen|
|Publication status||Published - 1 Dec 1997|
- Chemical analyses
- IR and Raman spectra
- Val Ferrera