Leonardsenite (IMA2011-059), with ideal formula MgAlF5(H2O)(2), is a new fumarole mineral from Eldfell volcano, Iceland. It has also been found in volcanic encrustations from the Hekla crater, Iceland. The mineral forms a soft and friable mass of white crystals up to 20 mu m in length. The streak is white and the luster is earthy. The calculated density is 2.31 g cm(-3). Leonardsenite is orthorhombic, space group Imma; the lattice parameters, obtained from Rietveld refinement of the XRPD data, are: a 7.055(1) A, b 10.117(2) angstrom, c 6.813(1) angstrom, V 486.3(1) angstrom(3), and Z = 4. The eight strongest reflections in the X-ray powder diffraction pattern are [d in A (I) (hkl)]: 5.66 (100) (011), 4.92 (29) (101), 3.53 (27) (200), 3.03 (31) (031), 3.00 (38) (211), 2.30 (16) (231), 1.77 (19) (400), 1.76 (24) (242). Chemical analyses by energy-dispersive spectrometry using a scanning electron microscope produced a mean elemental composition as follows (wt.: Mg 14.66, Al 16.16, F 52.98, 0 15.88, H 1.78, total 101.46. The corresponding empirical formula, calculated on the basis of 2 cations pfu, is Mgi.00Ali 004.64(011)0.361E5.00(1420)1 29. On the basis of chemical analyses and X-ray diffraction data, leonardsenite corresponds to the synthetic compound MgAlF5(H2O)(2). The crystal structure of leonardsenite contains infinite chains of [A(1)F(6)] octahedra along the c-axis which are connected via common fluorine atoms to isolated [MgF4(H2O)(2)] octahedra. Leonardsenite is the first aluminum fluoride hydrated mineral of the inverse weberite group with general formula (MMTIF5)-M-II-T-I(H2O)(2). Leonardsenite belongs to the group of fluorides with interesting optical and catalytic properties. The name of this new species honors Erik Leonardsen (1934), the former leader of the X-Ray Diffraction Laboratory of the Geological Institute, University of Copenhagen. The mineral and its name have been approved by the MA-CNMNC (IMA2011-059).