Crystal structure and low-temperature behavior of "disordered" thomsonite

The crystal structure, crystal chemistry, and low-temperature structural evolution of natural thomsonite from Terzigno, Somma-Vesuvius volcanic complex, Naples Province, Italy, have been investigated by means of in situ single-crystal X-ray diffraction, electron microprobe analysis in the wavelength dispersive mode, and Raman spectroscopy. Six structure refinements have been obtained at different temperatures: 295.5, 248.0, 198.0, 148.0, 98.0, and 296.0 K (alter the low-T experiments). The reflection conditions and the structure refinements prove that the crystal of thomsonite here investigated is orthorhombic with a = 13.0809(3), 0 = 13.0597(3), c = 6.6051(1) angstrom. V = 1128.37(14) angstrom(3), and space group Photo, which differs from thomsonite from different localities reported in previous studies (with a similar to 13.1, b similar to 13.06, c similar to 13.2 angstrom, and space group Pncn). The refined bond distances suggest that the Si/Al-distribution in the tetrahedral framework is fully "disordered," giving rise to the halving of the c axis relative to that found in "ordered" thomsonites. The extra-framework population consists of: (1) one site about 50% occupied by Ca (labeled as "Ca"); (2) one site occupied by Na (similar to 70%) and Ca (similar to 30%) (labeled as "Na"); and (3) three water molecule sites ("W1," "W2," "W3"). The structure refinements allowed the location of all the proton sites, and the hydrogen-bonding scheme in the structure is provided. The low-temperature refinements show no significant change in the structure within the T-range investigated. The evolution of the unit-cell volume with T exhibits a continuous and linear trend, without any evident thermo-elastic anomaly, with thermal expansion coefficients alpha(v) = v(-1).partial derivative V/partial derivative T = 20(2).10(-6) K(-1) (between 98.0 and 295.5 K). A list with the principal Raman active modes is provided and a comparison with the vibrational modes previously found for "ordered" thomsonite is carried out.