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

We relate here on the occurrence and the crystal structure of fantappièite (IMA 2008-006), 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 colourless crystals, showing complex morphologies and streaked faces; it is non-pleochroic and uniaxial negative, ω = 1.5046(5) and ε = 1.5027(5). Dcalc is 2.471 g/cm3. Fantappièite is trigonal, space-group P ; the cell parameters are (in Å): a = 12.8742(6), c = 87.215(3) Å, V = 12518.8(9) Å3, Z = 1. The empirical chemical formula is: (Na84.12Ca30.00K15.95Fe0.19Ti0.13Mn0.10Mg0.09)(Si99.36Al98.64)O396(SO4)30.24(CO3)0.29Cl0.84F0.82∙5.18H2O, which corresponds to the ideal formula [Na82.5Ca33K16.5]132(Si99Al99O396)(SO4)33· 6H2O. The strongest five reflections in the X-ray powder pattern are [d in Å (I %) (hkl)]: 3.70 (100) (0 3 0), 3.60 (80) ( 1 23), 2.641 (65) (0 0 33), 6.85 (60) ( 1 10), 6.40 (55) ( 2 0). The single-crystal FTIR spectrum rules out OH groups and shows the presence of H2O and CO2 molecules, as well as weak amounts of carbonate groups in the structural cages of the mineral. 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 ACBACABACBACBACBCACBACBACBABCBACB…, 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. Sulphate groups occur within the liottite cages associated by Na, K and Ca, while highly disordered sulphate 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 sulphate groups in the sodalite cages. The name fantappièite is for Acasto Liberto Fantappiè (Pontassieve, 12/04/1862 – Viterbo, 29/01/1933), a distinguished geologist and naturalist who dedicated part of his scientific activity to the mineralogy and petrology of the volcanic rocks of the Roman Comagmatic Region in the Sabatini area where the mineral has been found.