Högbomite is reported from two upper-amphibolite and granulite-facies, sapphirine-bearing, Al–Mg–Fe-rich and silica-poor lens-shaped layers within the Bamble Sector, south Norway. Primary assemblages, indicating peak metamorphic conditions of 773-844°C at 7 kbar (Mg–Fe exchange thermometry), are spinel–sapphirine–biotite–gedrite, spinel–corundum–sapphirine–cordierite and orthopyroxene–biotite–cordierite–plagioclase. Högbomite formed by hydrous alteration and oxidation of primary spinel and rutile and/or ilmenite according to the generalised reaction:

Suggested conditions of högbomite formation are 550–620°C and 6–7 kbar. The högbomites contain 10.2–14.7 wt.% MgO, 04).3 wt.% ZnO, 58.9–62.1 wt.% Al2O3 and 15.6-17.6 wt.% Fe as FeO. The two högbomite may belong to different polytypes, as suggested by their differing TiO2 (9.9-10.1 versus 5.7–5.8 wt. %) and calculated Fe3+ – and H2O–contents. The partitioning of Zn between spinel and högbomite is not uniform and is considered to depend upon prevailing fo2 and aH2O.

Dumortierite is described from several occurrences of cordierite-orthoamphibole-bearing rocks in the upper-amphibolite facies area of the Bamble Sector, south Norway. Dumortierite occurs with chlorite, muscovite and quartz in late M4 alteration zones or aggregates after M3, peak-metamorphic cordierite and garnet and early M4 vein-cordierite, and intergrown with or replacing orthoamphibole. Late M4P-T conditions, which are interpreted as conditions of dumortierite formation, are estimated from the associated late M4 kyanite-andalusite-chlorite-quartz assemblage and Mg-Fe exchange geothermometry to be 500 ± 50 °C and 3–4 kbar. Retrogression of M3 mineral assemblages is initiated by influx of fluids with XCO2 of 0.3–0.4 during early M4 followed by more water-rich fluids during late M4. Late M4 fluids may show local variations in alkalis and boron. The dumortierites are the most Mg-rich (2.23–3.42 wt. % MgO) ever reported, and contain 0.00–2.05 wt.% TiO2, 0.00–1.08 wt.% Fe2O3, 29.62–32.42 wt.% SiO2 and 55.20–59.71 wt.% Al2O3. Al is the most likely substituent for Si, which shows a minor deficiency at the tetrahedral sites in most dumortierites. The major variations in the mineral chemistry can be described by the coupled substitutions Mg + Ti = 2AlVI, 3Mg = 2AlVI and possibly Mg + H = AlVI. Favoured by low ƒO2 prevailing conditions a significant part of total iron in dumortierites at one locality is present as Fe2+ giving evidence for the Fe2+ + SiIV = AlIV + AlVI tschermakite substitution. FeMg−1 substitution is considered to be limited. Ti-rich dumortierites coexist with rutile, ilmenite or titaniferous magnetite. The development of dumortierite from orthoamphibole correlates with an observed decrease of Al, Mg and Na and increase of Si and Fe in orthoamphibole towards dumortierite.