Phlogopites in forsterite‐bearing marbles of the metasedimentary Faurefjell Formation in the high-grade Precambrian rocks of Rogaland, southwest Norway, contain up to 24.6 wt% BaO and up to 13.9 wt% TiO2. The very high TiO2 content and the low cation sums (Σcations < 16 and Si + Al < 8 per 22 oxygens) could be taken to indicate incorporation of tetrahedral Ti and/or Ti‐vacancy substitutions. However, combinations of Ti‐oxy and Ti‐vacancy substitution schemes explain the analytical data better. Structural formulae of the solid-solution trend have been expressed as approximately linear combinations of pure phlogopite and a Ba‐Ti component BaMg2TiSi2Al2O12, with small contributions of “eastonite” and Ti‐vacancy end‐member. The structural formula of the most Ba‐rich mica is (K0.14 Na0.05 Ba0.81)(Mg1.61Fe0.45Ti0.84Al0.060.03)Si2.13Al1.18O12((OH)0.38O.62), indicating that the unnamed anhydrous Ba‐Ti end‐member composition constitutes about 81 mol% of the solid solution.

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