Håleniusite-(Ce), CeOF, the Ce Analogue of Håleniusite-(La) from the Água de Pau Volcano, Sãn Miguel Island, Azores District, Portugal

Abstract

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 [d_(obs)Å(I_(obs))(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 Fm3̅ m, with a = 5.6597(10) Å and V = 181.29(10) ų (Z = 4).

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