Marinellite, a new feldspathoid of the cancrinite-sodalite group

Marinellite, [(Na,K)₄₂Ca₆](Si₃₆Al₃₆O144)(SO₄)₈Cl₂.6H₂O, cell parameters a = 12.880(2) Å, c = 31.761(6) Å, is a new feldspathoid belonging to the cancrinite-sodalite group. The crystal structure of a twinned crystal was preliminary refined in space group P31c, but space group P6̄2c could also be possible. It was found near Sacrofano, Latium, Italy, associated with giuseppettite, sanidine, nepheline, haüyne, biotite, and kalsilite. It is anhedral, transparent, colourless with vitreous lustre, white streak and Mohs' hardness of 5.5. The mineral does not fluoresce, is brittle, has conchoidal fracture, and presents poor cleavage on {001}. D_meas is 2.405(5) g/cm³, D_calc is 2.40 g/cm³. Optically, marinellite is uniaxial positive, non-pleochroic, ω = 1.495(1), ϵ = 1.497(1). The strongest five reflections in the X-ray powder diffraction pattern are [d in Å (I) (hkl)]: 3.725 (100) (214), 3.513 (80) (215), 4.20 (42) (210), 3.089 (40) (217), 2.150 (40) (330). The electron microprobe analysis gives K₂O 7.94, Na₂O 14.95, CaO 5.14, Al₂O₃ 27.80, SiO₂ 32.73, SO₃ 9.84, Cl 0.87, (H₂O 0.93), sum 100.20 wt %, less O = Cl 0.20, (total 100.00 wt %); H₂O calculated by difference. The corresponding empirical formula, based on 72 (Si + Al), is (Na_31.86K_11.13Ca_6.06)Σ=49.05(Si_35.98Al_36.02)Σ=72O_144.60(SO₄)_8.12C1_1.623.41H₂O. The crystal structure of marinellite may be described as formed by the stacking along c of 12 layers containing six-membered rings of tetrahedra: the stacking sequence is ABCBCBACBCBC…, where A, B, and C represent the positions of the rings within the layers. Its structure consists of two liottite cages superimposed along [0, 0,z] and of columns of cancrinite and sodalite cages along [1/3, 2/3, z] and [2/3, 1/3, z]. Sulphate groups, surrounded by sodium, calcium and potassium cations, occupy the liottite cages. Chlorine anions and sulphate groups occupy the sodalite cages, whereas H₂O molecules are located within the cancrinite cages, bonded to Na cations. This structural model was refined in the space group P31 c, conventional R = 0.098 for 2155 reflections. The structural relationships between marinellite and tounkite are discussed.