KOTTENHEIMITE, Ca3Si(OH)6(SO4)2{middle dot}12H2O, A NEW MEMBER OF THE ETTRINGITE GROUP FROM THE EIFEL AREA, GERMANY

The new ettringite-group mineral kottenheimite was found at Bellerberg, near Kottenheim, Eastern Eifel area, Rhineland- Palatinate (Rheinland-Pfalz), Germany, and named for the type locality. Associated minerals are wollastonite, clinochlore, ellestadite, melilite, cuspidine, and earlier formed sanidine, clinopyroxene, and magnetite. Kottenheimite forms white radiated and random aggregates of hair-like subparallel clusters of minute crystals. The Mohs’ hardness is 2–2.5; D meas = 1.92(2) g/cm 3 , D calc = 1.926 g/cm 3 . The new mineral is uniaxial (−), ω = 1.490(2), ɛ = 1.477(2). The IR spectrum of kottenheimite is in accordance with the chemical composition and contains absorption bands in the range 3300–3700 cm −1 (O–H-stretching vibrations), at 1650 and 1683 cm −1 (bending vibrations of H 2 O molecules), 1158, 1086, and 987 cm −1 (showing the presence of distorted SO 4 2− groups), 752 and 725 cm −1 (corresponding to Si–O stretching vibrations of Si(OH) 6 octahedra). The chemical composition is (electron microprobe, mean of 6 analyses, wt.%; H 2 O and CO 2 determined by gas chromatography): CaO 26.04, MgO 0.20, FeO 0.19, Al 2 O 3 0.25, SiO 2 8.95, SO 3 24.26, CO 2 0.58, H 2 O 41.30; total 101.77. The empirical formula calculated on the basis of 26 anions is Ca 3.015 Mg 0.03 Fe 0.02 Al 0.03 Si 0.97 (OH) 5.94 (SO 4 ) 1.97 (CO 3 ) 0.09 ·11.91H 2 O. The simplified formula is Ca 3 Si(OH) 6 (SO 4 ) 2 ·12H 2 O. The crystal structure was refined by the Rietveld method ( R p = 0.0487, R wp = 0.0623, R B = 0.087) based on the structural model of carraraite, Ca 3 Ge(SO 4 ,CO 3 ) 2 (OH) 6 ·12H 2 O. The new mineral is hexagonal, space group P 6 3 / m , a = 11.1548(3), c = 10.5702(3) A, V = 1139.04(5) A 3 , and Z = 2. The strongest lines of the powder diffraction pattern [ d in A ( I ) ( hkl )] are: 9.72 (100) (100), 5.590 (60) (110), 4.645 (26) (102), 3.840 (54) (112), 2.751 (34) (302), 2.536 (27) (213), 2.185 (30) (223). The channels in the crystal structure of kottenheimite are only filled by sulfate anions and water molecules. Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 4102/1.