The new mineral topsøeite, FeF3(H2O)3, was found as a fumarolic product after the 1991 eruption of Hekla, Iceland. The mineral occurs as up to 20 µm large square-prismatic crystals forming occasional stepped aggregates or massive, up to 100 µm wide veins, in association with several other fluorides, hematite and opal. The experimental formula of the mineral (from scanning electron microscope energy-dispersive spectrometry data) is Fe(F2.94Cl0.04)Σ2.98(H2O)1.94. The deficiency of water in the formula is most probably an artefact due to experimental limitations and not a sign of dehydration. The mineral is yellow, with a calculated density of 2.330 g·cm−3, based on the ideal formula. It is tetragonal (P4/n) with a = 7.8381(3) Å, c = 3.8674(1) Å, V = 237.60(2) Å3. The strongest eight powder diffraction lines are [d in Å (relative intensity) (hkl)]: 5.55 (100) (1 1 0); 3.92 (43) (0 2 0); 3.47 (39) (0 1 1); 3.17 (22) (1 1 1); 2.77 (30) (2 2 0); 2.479 (31) (1 3 0, 3 1 0); 1.877 (16) (0 1 2), 1.753 (24) (2 4 0, 4 2 0). Rietveld refinement of the powder diffraction data confirmed the identity of topsøeite with synthetic β-FeF3(H2O)3. The crystal structure consists of straight infinite chains of [FeF4(H2O)2] octahedra extending along the c axis. The adjacent octahedra share apical F atoms, whereas the four unshared, equatorially coordinated atoms are represented by a disordered arrangement of two F and two O atoms from water molecules. Additional water molecules occupy the spaces between chains and are tetrahedrally coordinated by four (F, H2O) from four different chains binding them together via hydrogen bonds. Topsøeite is isostructural with rosenbergite, AlF3(H2O)3. Both minerals have rhombohedral polymorphs known from studies of phase systems. The polymorph of topsøeite (UM2008-27-F:AlHO), earlier supposed to be aluminium fluoride hydrate, was also found in Hekla fumaroles from the 1991 eruption, but its genetic relation with topsøeite remains unclear. Topsøeite is named after the family of Danish prominent scientists and industrialists including Haldor Topsøe the elder (1842–1935), Haldor Topsøe the younger (1913–2013) and Henrik Topsøe (1944).

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