Abstract: Black, opaque grains of a spinel whose composition is (Mg₂TiO₄) 85.8, (MgFe₂O₄) 0.4 (FeFe₂O₄) 13.8 (mole %) coexist with a MgAl₂O₄ spinel and geikielite in a periclase-forsterite marble that has been thermally metamorphosed against an alkalic ultramafic intrusion of Caledonian age in the Kangerdlugssuaq region of East Greenland. The spinel appears to be the closest recorded approach to the end-member Mg₂TiO₄ among natural rocks, and to be part of a solid-solution series extending across the join Mg₂TiO₄-MgFe₂O₄-FeFe₂O₄, the existence of which has not previously been reported. The composition of the series appears to be controlled by the f_O2 that prevails during metamorphism.