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

kircherite (IMA 2009-084), a new member of the cancrinitesodalite group of minerals from Valle Biachella, Sacrofano, (Rome), in the Sabatini volcanic complex, Latium, Italy. The mineral occurs within miarolitic cavities of holocrystalline, syenitic, volcanic ejecta, associated with sodalite, biotite, iron oxides, titanite, fluorite, and a pyrochlore group mineral. The groundmass of the ejecta consists essentially of potasiumfeldspar, with minor sodalite and brown mica. Kircherite occurs typically as parallel associations of hexagonal, thin, tabular crystals (2 or 3 mm in diameter and 1 mm in thickness). Single platelets have a thickness that very rarely exceeds 0.5 mm. Crystals are colourless to light gray to white, translucent to opaque in the most altered parts of the crystals; lustre is greasy to silky. Fluorescence is in light pink under LW UV and deep red under SW UV. Hardness is (Mohs): 5.5; micro-indentation VHN load 10 g, mean 648.4 kg mm-2, range ± 208.9 kg mm-2. Cleavage on {001} is good. Density (calc.) is 2.457 g cm-3. Kircherite is non-pleochroic and uniaxial negative with ω = 1.510(2), ε = 1.502(2). Kircherite is trigonal, space group R32, with , a = 12.8770(7) Å, c = 95.224(6) Å, V = 13677.2(13) Å3, Z = 1. The empirical chemical formula obtained from EMP analyses is: (Na89.09Ca31.63K18.85Fe0.20Mn0.06Mg0.05Ti0.03)Σ=139.91 (Si108.13 Al107.87)Σ=216.00O430.08(SO4)32.59 Cl2.00F0.53(CO3)0.09·6.86H2O, which corresponds to the ideal formula [Na90Ca36K18]Σ=144 (Si108Al108O432)(SO4)36·6H2O. The strongest six reflections in the X-ray powder pattern are [d in Å (I %) (hkl)]: 3.72 (100) (300), 2.65 (100) (2.1.28; 0.0.36), 3.23 (65) (2.1.19), 3.58 (60) (1.2.14), 3.60 (53) (1.0.25), 3.80 (52) (1.2.11). 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. The structure can be described as a stacking sequence of 36 layers of six-membered rings of tetrahedra along the c axis as ACABCABCABCACBCABCABCABCBABCABCABCAB…, where A, B and C represent the positions of the rings within the layers. The structure has 6 cancrinite cagϵe)s, 2(4 sodalite cages (S) and 6 LOSOD cages (L) within the unit cell, and these account for a maximum of 36 SO4 2- groups and 6 H2O molecules (or Cl anions). There is a unique sequence of cages, ϵSSSLSSLSSSϵ and adjacent sequences are shifted 1/3 along [001]. Kircherite, having 36 layers, is the cancrinite with the longest complex sequence described to date. The name is for Athanasius Kircher (1602–1680), a German Jesuit who was the founder of the museum of the Collegium Romanum in 1651, hereafter named the Museum Kircherianum.