Håleniusite-(Ce) (IMA2021-042), CeOF, is a new mineral discovered in ejectum from the Água de Pau volcano, Sãn Miguel Island, Azores District, Portugal. It occurs as opaque, cream-colored, fine-grained pseudomorphic replacements of hexagonal tablets of bastnäsite-(Ce). It is associated with astrophyllite and fluornatropyrochlore in a matrix composed mainly of albite, quartz, and aegirine. The mineral has a white streak and a calculated density of 5.890 g/cm3 for the empirical formula. The strongest powder diffraction lines are [dobsÅ(Iobs)(hkl)]: 3.247(100)(111), 2.840(31)(200), 2.004(46)(220), and 1.705(42)(311). Electron probe microanalysis provided the empirical formula (Ce0.41La0.21Sm0.15Nd0.04Eu0.03Ca0.02Y0.02Dy0.02Gd0.01)Σ0.91(O0.70F0.30)F1.00. Håleniusite-(Ce) has a fluorite-type structure, space group Fmforumlam, with a = 5.6597(10) Å and V = 181.29(10) Å3 (Z = 4).

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