The occurrence of preiswerkite in a tourmaline-biotite-scapolite rock fromBlengsvatn, Norway

We report paragenesis and chemistry of a new occurrence of the rare triocta hedral Al-rich sodium mica preiswerkite. The preiswerkite occurs in a tourm waline-biotite-scapolite rock in the contact zone of a gabbroic boudin surr ounded by Proterozoic metasediments near the Blengsvatn, Bamble sector, sou thern Norway. The preiswerkite occurs as subhedral crystals or is intergrow n with biotite in a polygonal fabric together with Cl-rich scapolite + tour maline +/- ilmenite +/- plagioclase +/- corundum. Accessory minerals are he matite, hogbomite, spinel, allanite, apatite, and zircon, with relic calcit e. Preiswerkite has the compositional range: (Na1.84-2.02K0.02-0.10Caless than or equal to 0.04)((Mg3.13-3.42Fe0.63-0.77 Al1.87-2.07)-Al-VI)((Al3.58-3.96Si4.04-4.29Sless than or equal to 0.02)-Al- IV)O-20[Cl-less than or equal to 0.03(OH)(greater than or equal to 3.97)] a nd coexists with Na-Al-rich biotite, with the composition: (K1.38-1.61Na0.18-0.45Caless than or equal to 0.03)((Mg3.72-3.88Fe1.38-1.43 Ti0.10-0.16Al0.63-0.85)-Al-VI)((Al2.71-2.93Si5.07-5.29)-Al-IV)O-20[Cl-0.02( OH)(greater than or equal to 3.98)]. We suggest that the assemblage preiswerkite + biotite + tourmaline + scapol ite +/- ilmenite +/- plagioclase +/- corundum was formed during prograde or peak metamorphism in the area, at similar to 700 degrees C and 7 kb.