Fe-dominant rhönite with between 25 and 28 wt% FeO (as total iron) occurs as a late crystallizing phase in tephrite and limburgite from the Kaiserstuhl volcanic complex, Upper Rhine Graben, Germany. Average compositions are: (K0.05Na0.40Ca1.55)2 (Mg1.85Mn0.03Fe2+2.45Fe3+0.66Al0.09Ti0.92)6(Si3.88Al2.12)6O20 in tephrite and (K0.01Na0.58Ca1.41)2(Mg1.93Mn0.03Fe2+2.26Fe3+0.71 Al0.17Ti0.90)6(Si3.96Al2.04)6O20 in limburgite. Mg/(Mg + Fe2+) ratios range between 0.40 and 0.49, and the Fe2+-rich rhönite can be classed as an unnamed Fe2+-dominant analogue of rhönite with XMg <0.5 as defined by Grew et al. (2008). Compared with other Fe2+-dominant rhönites in terrestrial and extraterrestrial rocks, the Kaiserstuhl compositions have less Ti and are richer in Si + (Na,K). The Kaiserstuhl rhönite coexists with subsilicic Al–Ti-bearing augite and titanian magnetite as in other basaltic occurrences of Fe2+-dominant and Mg-dominant rhönite, and is a quench phase in glass with liquidus temperatures of 1137–1068 °C.

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