New data on marinellite, a mineral of the cancrinite-sodalite group

Marinellite, a cancrinite-group mineral with ideal composition [(Na,K)₄₂Ca₆](Si₃₆Al₃₆O₁₄₄)(SO₄)₈Cl₂₆•H₂O was reported for the firts time (Bonaccorsi and Orlandi, 2003) from Valle Biachella, Sacrofano (Roma, Italy). The crystal structure of marinellite [<i>P</i>31<i>c</i> with <i>a</i> = 12,880(2) Å and <i>c</i> = 31,761(6) Å] is characterized by columns of liottite cages running along [0, 0, <i>z</i>] and columns of cancrinite + sodalite cages staked along [1/3, 2/3, <i>z</i>] and [2/3, 1/3, <i>z</i>]. Na⁺, Ca²⁺ and K⁺, SO4^2- and Cl^- are hosted within the liottite cages, while additional H₂O and Na⁺ are hosted within the cancrinite cages. We describe here a new occurrence of marinellite from Magliano Romano, Sabatini Volcanic Complex north of Rome. The mineral is found as millimetric-sized, transparent to trasluncent, vitreous, light-blue colored, anhedral grains, within holocrystalline volcanic ejecta associated with biotite, clinopyroxene, and Fe-oxides. Powder diffraction data were collected with a Scintag X1 diffractometer (CuK_α, solid state detector); EMP analyses were done using a CAMEBAX 50X WDS-microprobe (15 kV accelerating voltage, 4 nA beam current, spot size 20 μm). Powder-FTIR spectra were collected on KBr pellets with a spectrometer Nicolet Magna 760 (KBr beamsplitter, DTGS detector, Globar source); polarized micro-FTIR single crystal spectra were collected on oriented, doubly-polished slabs, using a NicPlan microscope equipped with a nitrogen-cooled MCT detector and a ZnSe wire-grid IR polarizer. The cell parameters: <i>a</i> = 12,881(1) Å, <i>c</i> = 31,764(4) Å, V = 4564,3(7) ų, and the chemical formula of marinellite from Magliano Romano: (Na_29.97K_11.57Ca_6.55)_=48.09(Si_35.93Al_36.07)_=72.00O₁₄₄[(SO₄)_7.93Cl_1.65(CO₃)_0.52]_=10.11•5.77H₂O, are close to those of the holotype specimen from Valle Biachella. Powder-FTIR spectra show the absorptions due to H₂O (₁/₃ at 3500 cm^-1 and ₂ at 1630 cm^-1), SO₄ (₃ at  1100 cm^-1) and the alumino-silicate framework ( 1000 cm^-1 and below). Single crystal polarized FTIR spectra shows the absorptions due to CO₃ groups (₃  1400 cm^-1), H₂O molecules, and two very unusual absorptions, due to molecular CO₂ (Della Ventura et al. 2005, 2007a, 2007b), at 2352 and 2341 cm^-1. The first component is almost completely polarized for E  c, while the second one has an isotropic behaviour. The polarization behavior of H₂O stretching and bending bands is quite complex and shows the presence of two different water molecules within the structural channels, in agreement with the presence of two bending absorptions at around 1630 cm^-1. References Bonaccorsi E., Orlandi P. (2003) Eur. J. Mineral., 15, 1019-1027. Della Ventura, G., Bellatreccia, F. and Bonaccorsi, E. (2005) Eur. J. Mineral., 17, 847-851. Della Ventura G., Bellatreccia F., Parodi G.C., Cámara F., Piccinini M. (2007a) Am. Mineral. (in press). Della Ventura G., Bellatreccia F., Piccinini M. (2007b) Rend. Fis. Acc. Lincei (in press).