Kristjánite, KNa2H(SO4)2, a new fumarolic mineral from Iceland containing [SO4-H-SO4]3-anion in the crystal structure

Kristjánite, KNa2H(SO4)2, is a new mineral found in a high-temperature fumarole on Fimmvör∂uháls, Iceland. It is monoclinic, P21/n, a, b, c (Å) = 6.9625(1), 9.9953(1), 11.0928(2), β = 105.637(2)°, V = 743.40(2) Å3. Kristjánite forms colourless transparent crystals up to about 100 μm in size in compact aggregates with metathénardite, belomarinaite, aphthitalite, ivsite and an unknown mineral with tentative composition K2NaH(SO4)2, or grows in white globules of smaller grains on their surface. The eight strongest maxima in a Powder X-Ray Diffraction diagram are [d(Å)Intensity] 4.376, 3.653, 3.488, 3.3610, 3.188, 2.834, 2.734and 2.4052. The mineral represents a novel crystal structure type. In it, K is coordinated by eight O atoms, and two symmetrically independent Na atoms by seven O atoms. Two symmetrically independent S atoms are in tetrahedral coordination by O atoms. A close to linear, very short hydrogen bond (2.44 Å), connects the two to a SO4-H-SO4dimer. K and Na coordination polyhedra share vertices, edges and even faces forming with sulphate tetrahedra a tight structure with narrow [100] channels lined on two opposite sides by hydrogen bonds.