Kruijenite, Ca4Al4(SO4)F2(OH)16·2H2O, a new mineral with microporous structure from the Eifel paleovolcanic region, Germany
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The new mineral kruijenite, ideally Ca4Al4(SO4)F2(OH)16·2H2O, was found in a calcic xenolith from tephra of the Feuerberg paleovolcano situated near Daun, Eifel paleovolcanic region, Rhineland-Palatinate, Germany. It is associated with fluorite, calcite, aragonite, cuspidine, magnesioferrite, hematite, sharyginite, harmunite, and an insufficiently investigated hydrous Ca-Mg-Al silicate. Kruijenite occurs as pale greenish-yellow to colourless long prismatic tetragonal crystals up to 0.1 mm × 1 mm in cavities typically combined in radiating or random aggregates. The mineral is brittle, with Mohs’ hardness of 3; Dcalc is 2.573 g/cm3. The IR spectrum is given. Kruijenite is optically uniaxial (−), ω = 1.576(3), ε = 1.561(3). The chemical composition (wavelength dispersive spectrometer Oxford INCA Wave 700 EPMA analyser, H2O calculated from structural data) is: CaO 32.38 wt%, Al2O3 27.75 wt%, Cr2O3 1.45 wt%, SO3 8.09 wt%, F 5.84 wt%, H2O 25.64 wt%, –O=F –2.46 wt%, total 98.69 wt%. The empirical formula is Ca4.00(Al3.77Cr0.13)Σ3.90(SO4)0.70F2.13(OH)16.17·1.79H2O (Z = 2). Kruijenite is tetragonal, space group P4/ncc, a = 12.9299(4) Å, c = 5.2791(3) Å, V = 882.57(6) Å3, Z = 2. The crystal structure was solved and refined to R = 0.121. Kruijenite represents a novel structure type. Its structure is based on the microporous pseudo-framework built by Al(OH)6 octahedra and CaF2(OH)6 polyhedra. The strongest reflections of the powder X-ray diffraction pattern [d (I) (hkl)] are: 9.12 Å (77%) (110), 4.565 Å (100%) (220), 4.084 Å (50%) (310), 2.964 Å (74%) (321), 2.694 Å (27%) (411), 2.321 Å (24%) (431), 2.284 Å (29%) (511), 2.217 Å (22%) (321, 530), 1.971 Å (40%) (611). The mineral is named in honor of the Dutch collector of Eifel minerals Fred Kruijen (born in 1956).
KeywordsKruijenite New mineral Hydrous calcium aluminium fluorosulfate Microporous crystal structure Calcic xenolith Feuerberg quarry Eifel
Constructive comments of two anonymous reviewers, handling Associate Editor Luca Bindi, and Editor-in-Chief Lutz Nasdala are gratefully acknowledged. This work was partly performed in accordance with the state task, state registration No. 0089-2016-0001. This work was financially supported by the Russian Foundation for Basic Research, grant nos. 18-29-12007-mk (single crystal and powder X-ray diffraction data, determination of the chemical composition and crystal structure) and 18-55-18003 (physical properties, including IR spectroscopy). The technical support by the St. Petersburg State University X-Ray Diffraction Research Resource Center in the powder Xray diffraction studies is acknowledged.
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