Carbon-bearing cordierite from Allumiere (Tolfa volcanic center, Latium, Italy): Occurrence, crystal-structure and FTIR microspectroscopy

This paper reports the crystalchemical study of cordierite from a volcanic ejectum from the Tolfa volcanic center (Latium, Italy). The studied rock was sampled within the pyroclastic deposit (“tufo caotico”) outcropping in a quarry at Colle dello Spizzicatore, Allumiere Community. The sample was studied by electron microprobe analysis (EMP), single-crystal structure refinement and Fourier-transform infrared microspectroscopy (FTIR). Combination of these techniques allowed detailed characterisation of the amount, location and coordination of cations and volatile molecules in the channels. Cordierite from Allumiere is orthorhombic, space group Cccm; refined cell-parameters are (in Å): a = 17.1580(6), b = 9.7573(3) and c = 9.3228(3). EMP analyses show an intermediate XMg [Mg/(Mg+Fe) = 0.58], and a very low alkali (Na + K = 0.042 apfu) content. Single-crystal FTIR spectra are strongly polarised; the absorptions observed in the H2O stretching region can be assigned to H2O molecules oriented with the H…H vector // a and the molecular plane // (001). The absorptions observed in the C-O stretching region can be assigned to both CO2 and CO molecules oriented // a; the spectra also show the presence of 13C and 18O in the sample. Using the molar absorption coefficients available in the literature, H2O = 0.02 wt% and CO2 = 0.22 wt% are calculated from the measured band intensities. The observed mineral assemblage and the volatile composition of cordierite are coherent with a thermometamorphic overprint on an early metapelite, which occurred at low-P (2 Kbar), high-T (> 700°C) and extremely reducing conditions.