Farneseite, a new mineral of the cancrinite - sodalite group with a 24-layer stacking sequence : occurence and crystal structure

This paper reports the occurrence and crystal structure of farneseite, a new member of the cancrinite-sodalite group from Farnese (Viterbo province, Latium, Italy). The mineral was observed within a syenitic ejected block consisting of dominant sanidine with minor andradite, clinopyroxene (augite), biotite, Fe-oxides and feldspathoids. Farneseite (0.5 min in longest dimension) occurs in the vugs of interlocking feldspars, as transparent colourless crystals, showing complex hexagonal morphologies and streaked faces; it is non-pleochroic and uniaxial positive, ω = 1.499(1) and ε = 1.503(1). Dcalc, is 2.425 g/cm3. Farneseite is hexagonal, space-group P63/M; the cell parameters are (in Å): a 12.8784(2), c = 37.0078(12) Å, V = 5315.54(2) Å3, Z = 1. The chemical formula is: (Na36.43K9.8 Ca8.75)∑=54.36(Si42.50Al41.50) O168(SO4)11.43F0.16 Cl0.48·3.03H2O; the ideal cell formula is [(Na,K)46Ca10]∑=56(Si42 Al42O168)(SO4) 12·6H2O. The strongest five reflections in the X-ray powder pattern are [d in Å (I) (hkl)]: 3.722 (100) (300), 3.485 (65) (216), 3.119 (36) (218), 2.149 (34) (330; 4 0 11), 2.648 (32) (228; 0 0 14). The single-crystal FTIR spectrum rules out OH groups and shows the presence of H2O and minor CO2 molecules, as well as weak amounts of carbonate groups in the structural channels of the mineral. The structure can be described as a stacking sequence of 14 layers of six-membered rings of tetrahedra along the c axis. The stacking sequence is ABCABABACBACAC..., 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 surrounded by Na, K and Ca, while highly disordered sulphate groups are located within the sodalite cages. H2O molecules 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.