A new mineral, eldfellite, was found among fumarolic encrustations collected in 1990 on the Eldfell volcano, Heimaey Island, Iceland. Associated minerals are ralstonite, anhydrite, gypsum, bassanite, hematite, opal and tamarugite, as well as a presumably new mineral with the composition Na3Fe(SO4)3. Along with opal and tamarugite, eldfellite forms soft and fragile aggregates built of thin, platy crystals of micrometre size. The mineral is yellowish-green to greenish-white, with a white streak. The calculated density is 3.062 g/cm3. Eldfellite is monoclinic, C2/m, a 8.043(4) Å, b 5.139(2) Å, c 7.115(4) Å, β 92.13(2)°, Vuc 293.9(2) Å3, Z = 2 and is isostructural with yavapaiite [KFe(SO4)2]. The strongest lines in the powder diffraction diagram are [d (Å), I (relative to 10)]: 3.72, 8; 3.64, 5; 3.43, 5; 2.77, 10; 2.72, 6; 2.57, 3; 2.370, 6; 1.650, 3. The chemical analysis and the X-ray diffraction data of eldfellite correspond to those of the synthetic compound NaFe(SO4)2.

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