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 ω = 1.510(2) and ɛ = 1.502(2). Dcalc is 2.457 g/cm3. Kircherite is trigonal with a = 12.8770(7), c = 95.244(6) Å, V = 13677(1) Å3, 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/σI > 2. The strongest seven reflections in the X-ray powder pattern are [d in Å (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 H2O and CO2 molecules in the structural cages of the mineral. Chemical analysis gives (in wt%): SiO2 32.05, Al2O3 27.13, FeO 0.07, K2O 4.38, CaO 8.75, Na2O 13.62, MgO 0.01, MnO 0.02, TiO2 0.01, SO3 12.87, Cl 0.35, F 0.05, total 99.82. The empirical formula calculated on the basis of ∑(Si+Al) = 216 apfu is (Na89.09Ca31.63K18.85Fe0.20Mn0.06Mg0.05Ti0.03)∑=139.91 [(Si108.13Al107.87)∑=216.00O430.00](SO4)32.58Cl2.00F0.53·6.86H2O, which corresponds to the ideal formula [Na90Ca36K18]∑=144(Si108Al108O432)(SO4)36·6H2O. 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 (SO42−) in sodalite cages show positional disorder, and so do consequently the extraframework cation sites related to them.