Marinellite, a new feldspathoid of the cancrinite-sodalite group

摘要

Marinellite, (Na, K)_(42)Ca_6(Si_(36)Al_(36)O_(144))(SO_4)_8Cl_2·6H_2O, 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 P31c, but space group P6-bar2c could also be possible. It is anhedral, transparent, coloruless with vitreous lustre, white streak and Mohs' sanidine, nepheline, hauyne, 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/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_2O 7.94, Na_2O 14.95, CaO5.14, Al_2O_3 27.80, SiO_2 32.73, SO_3 9.84, C10.87, (H_2O0.93), sum 100.20 wt, less O = Cl0.20, (total 100.00 wt); H_2O ca.culated 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=72)O_(144.60)(SO_4)_(8.12)Cl_(1.62)·3.41H_2O. 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 catins, occupy the liottite cages. Chlorine anions and sulphate groups occupy the sodalite cages, whereas H_2O molecules are located within the cancrinite cages, bonded to Na cations. This structural model was refined in the space group P31c, conventional R = 0.098 FOR 2155 reflections. The structural relationships between marinellite and tounkite are discussed.