Fantappièite, a new mineral of the cancrinite-sodalite group with a 33-layer stacking sequence : occurrence and crystal structure

This paper reports the occurrence and the crystal structure of fantappièite, a new member of the cancrinite-sodalite group of minerals from Torre Stracciacappe, Trevignano community (Rome, Latium, Italy). The mineral occurs within a volcanic ejectum consisting of dominant sanidine with minor plagioclase, biotite, augitic clinopyroxene, andradite, and iron oxides. Fantappièite (0.7 mm as largest size) is observed within miarolitic cavities of the rock as transparent colorless crystals, showing complex morphologies and striated faces. It is non-pleochroic and uniaxial negative, nω = 1.5046(5) and nε = 1.5027(5). D calc is 2.471 g/cm3. Fantappièite is trigonal, space group P3̄; the cell parameters are: a = 12.8742(6), c = 87.215(3) Å, V = 12518.8(9) Å3, Z = 1. The empirical chemical formula is: (Na84.12Ca30.00K15.95Fe0.19Ti 0.13Mn0.10Mg0.09)(Si99.36Al 98.64)O396 (SO4)30.24 (CO 3)0.29Cl0.84F0.82·5.18H 2O, which corresponds to the ideal formula [Na82.5Ca 33K16.5]Σ=132 (Si99Al 99O396) (SO4)33·6H 2O. The five strongest reflections in the X-ray powder pattern are [d in Å (I %) (hkl)]: 3.70 (100) (3 0 0), 3.60 (80) (1 0 23), 2.641 (65) (0 0 33), 6.85 (60) (0 1 10), 6.40 (55) (1 1 0). The single-crystal FTIR spectrum rules out OH groups and shows the presence of H2O and CO2 molecules, as well. The structure can be described as a stacking sequence of 33 layers of six-membered rings of tetrahedra along the c axis. The stacking sequence is ACBACABACBACBACBCACBACBACBAB-CBACB⋯, where A, B, and C represent the positions of the rings within the layers. This sequence gives rise to liottite, sodalite, and cancrinite cages, alternating along c. Sulfate groups occur within the liottite cages associated by Na, K, and Ca, while highly disordered sulfate groups are located within the sodalite cages. H2O groups occur within the cancrinite cages, bonded to Ca and Na cations. Split positions are found for Na-Ca sites, and are related to disordering of the sulfate groups in the sodalite cages.