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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 ε = 1.502(2). D calc  is 2.457 g/cm 3 . Kircherite is trigonal with a = 12.8770(7), c = 95.244(6) Å, V = 13677(1) Å 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/σI &gt; 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 H 2 O and CO 2  molecules in the structural cages of the mineral. Chemical analysis gives (in wt%): SiO 2  32.05, Al 2 O 3  27.13, FeO 0.07, K 2 O 4.38, CaO 8.75, Na 2 O 13.62, MgO 0.01, MnO 0.02, TiO 2  0.01, SO3 12.87, Cl 0.35, F 0.05, total 99.82. The empirical formula calculated on the basis of Σ(Si+Al) = 216 apfu is (Na 89.09 Ca 31.63 K 18.85 Fe 0.20 Mn 0.06 Mg 0.05 Ti 0.03 ) Σ=139.91  [(Si 108.13 Al 107.87 ) Σ=216.00 O 430.00 ](SO 4 ) 32.58 Cl 2.00 F 0.53 ⋅6.86H 2 O, which corresponds to the ideal formula [Na 90 Ca 36 K 18 ] Σ=144 (Si 108 Al 108 O 432 )(SO 4 ) 36 ⋅6H 2 O.

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