Kircherite, a new mineral of the cancrinite-sodalite group with a 36-layer stacking sequence: Occurrence and crystal structure.

This paper reports on the occurrence and the crystal structure of kircherite, a new member of the cancrinite-sodalite group of minerals from Valle Biachella, Sacrofano community (Rome, Latium, Italy). The mineral occurs in association with sodalite, biotite, iron oxides, titanite, fluorite, and a pyrochlore-group mineral. The groundmass of the ejectum consists essentially of K-feldspar with subordinate plagioclase. Kircherite (3 mm as largest size) is observed within miarolitic cavities of the rock and typically occurs as parallel associations of hexagonal, thin, tabular colorless to light-gray transparent crystals; it is non-pleochroic and uniaxial negative, with ? = 1.510(2) and = 1.502(2). Dcalc is 2.457 g/cm3. Kircherite is trigonal with a = 12.8770(7), c = 95.244(6) A, V = 13677(1) A 3, Z = 1. The structure has been refined in the trigonal space group R32, obtaining a R-value of 8.5% on 8131 reflections with I/δI > 2. The strongest seven reflections in the X-ray powder pattern are [d in A (I %) (hkl)]: 3.717 (100) (3 0 0), 2.648 (100) (2 1 28; 0 0 36), 3.232 (65) (2 1 19), 3.584 (60) (1 2 14), 3.604 (53) (1 0 25), 3.799 (52) (1 2 11), 3.220 (38) (2 2 0). The single-crystal FTIR spectrum rules out OH groups and shows the presence of H 2O and CO 2 molecules in the structural cages of the mineral. Chemical analysis gives (in wt%): SiO2 32.05, Al 2O 3 27.13, FeO 0.07, K 2O 4.38, CaO 8.75, Na 2O 13.62, MgO 0.01, MnO 0.02, TiO 2 0.01, SO 3 12.87, Cl 0.35, F 0.05, total 99.82. The empirical formula calculated on the basis of ∑(Si+Al) = 216 apfu is (Na 89.09Ca 31.63K 18.85Fe 0.20Mn 0.06Mg 0.05Ti 0.03)∑= 139.91[(Si 108.13Al 107.87)∑= 216.00O 430.00](SO 4) 32.58Cl 2.00F 0.536.86H 2O, which corresponds to the ideal formula [Na 90Ca 36K 18]∑= 144(Si 108Al 108O 432)(SO 4) 36?6H 2O. The structure can be described as a stacking sequence of 36 layers of six-membered rings of tetrahedra along the c axis. The stacking sequence is ACABCABCABCACBCABCABCABCBABCABCABCAB, where A, B, and C represent the positions of the rings within the layers. This sequence gives rise to cancrinite, sodalite, and losod cages, alternating along c. Sulfate groups occur within the sodalite and losod cages associated by Na, K, and Ca. H2O groups occur within the cancrinite cages, bonded to Ca and Na cations. Anion groups (SO 4 2-) in sodalite cages show positional disorder, and so do consequently the extraframework cation sites related to them.