Marinellite, a new feldspathoid of the cancrinite-sodalite group

Abstract

Marinellite, [(Na,K) 42 Ca 6 ](Si 36 Al 36 O144)(SO 4 ) 8 Cl 2 .6H 2 O, cell parameters a = 12.880(2) A, c = 31.761(6) A, is a new feldspathoid belonging to the cancrinite-sodalite group. The crystal structure of a twinned crystal was preliminary refined in space group P 31 c , but space group P 62 c could also be possible. It was found near Sacrofano, Latium, Italy, associated with giuseppettite, sanidine, nepheline, hauyne, biotite, and kalsilite. It is anhedral, transparent, colourless with vitreous lustre, white streak and Mohs9 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/cm 3 , D calc is 2.40 g/cm 3 . 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 A (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 K 2 O 7.94, Na 2 O 14.95, CaO 5.14, Al 2 O 3 27.80, SiO 2 32.73, SO 3 9.84, Cl 0.87, (H 2 O 0.93), sum 100.20 wt %, less O = Cl 0.20, (total 100.00 wt %); H 2 O calculated by difference. The corresponding empirical formula, based on 72 (Si + Al), is (Na 31.86 K 11.13 Ca 6.06 )Σ=49.05(Si 35.98 Al 36.02 )Σ=72O 144.60 (SO 4 ) 8.12 C1 1.62 3.41H 2 O. 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 H 2 O molecules are located within the cancrinite cages, bonded to Na cations. This structural model was refined in the space group P 31 c , conventional R = 0.098 for 2155 reflections. The structural relationships between marinellite and tounkite are discussed.