Kruijenite, Ca4Al4(SO4)F2(OH)16·2H2O, a new mineral with microporous structure from the Eifel paleovolcanic region, Germany

• Published in: Mineralogy and petrology 2019-04, Vol.113 (2), p.229-236
• Main Authors: Chukanov, Nikita V., Zubkova, Natalia V., Blass, Günter, Pekov, Igor V., Varlamov, Dmitry A., Belakovskiy, Dmitriy I., Ksenofontov, Dmitry A., Britvin, Sergey N., Pushcharovsky, Dmitry Yu
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Aragonite; Calcite; Calcium; Chemical composition; Crystal structure; Crystals; Diffraction patterns; Earth and Environmental Science; Earth Sciences; Empirical analysis; Fluorite; Frameworks; Geochemistry; Haematite; Hematite; Inorganic Chemistry; Magnesium; Mineralogy; Minerals; Organic chemistry; Original Paper; Polyhedra; Silicates; Sulfur trioxide; Sulphates; Tephra; Water hardness; Wavelength; X ray powder diffraction; X-ray diffraction
• ISSN: 0930-0708; 1438-1168
• DOI: 10.1007/s00710-019-00653-3

The new mineral kruijenite, ideally Ca 4 Al 4 (SO 4 )F 2 (OH) 16 ·2H 2 O, 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; D calc is 2.573 g/cm 3 . 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, H 2 O calculated from structural data) is: CaO 32.38 wt%, Al 2 O 3 27.75 wt%, Cr 2 O 3 1.45 wt%, SO 3 8.09 wt%, F 5.84 wt%, H 2 O 25.64 wt%, –O=F –2.46 wt%, total 98.69 wt%. The empirical formula is Ca 4.00 (Al 3.77 Cr 0.13 ) Σ3.90 (SO 4 ) 0.70 F 2.13 (OH) 16.17 ·1.79H 2 O ( Z  = 2). Kruijenite is tetragonal, space group P 4/ ncc , a =  12.9299(4) Å, c  = 5.2791(3) Å, V  = 882.57(6) Å 3 , 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 CaF 2 (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).