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Farneseite, a new mineral of the cancrinite - sodalite group with a 14-layer stacking sequence

¹ CNR-Istituto di Geoscienze e Georisorse, sezione di Pavia, via Ferrata 1, I-27100 Pavia, Italy
² Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy

^* E-mail: dellaven{at}uniroma3.it

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 mm 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, = 1.499(1) and = 1.503(1). D_calc is 2.425 g/cm³. Farneseite is hexagonal, space-group P6₃m; the cell parameters are (in Å): a = 12.8784(2),c = 37.0078(12) Å, V = 5315.54(2) ų, Z = 1. The chemical formula is: (Na_36.43K_9.18Ca_8.75)=_54.36(Si_42.50Al_41.50)O₁₆₈ (SO₄)_11.43F_0.16Cl_0.48.3.03H₂O; the ideal cell formula is [(Na,K)₄₆Ca₁₀]₌₅₆ (Si₄₂Al₄₂O₁₆₈)(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; 4011), 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 sixmembered 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.

Key-words: farneseite, new mineral, cancrinite group, microchemistry, crystal structure, FTIR spectroscopy..

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