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 (after the low-m>Tm> experiments). The reflection conditions and the structure refinements prove that the crystal of thomsonite here investigated is orthorhombic with m>am> = 13.0809(3), m>bm> = 13.0597(3), m>cm> = 6.6051(1) Å, m>Vm> = 1128.37(14) Å3, and space group m>Pbmnm>, which differs from thomsonite from different localities reported in previous studies (with m>am> ~ 13.1, m>bm> ~ 13.06, m>cm> ~ 13.2 Å, and space group m>Pncnm>). The refined bond distances suggest that the Si/Al-distribution in the tetrahedral framework is fully “disordered,” giving rise to the halving of the m>cm> 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 (~70%) and Ca (~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 m>Tm>-range investigated. The evolution of the unit-cell volume with m>Tm> exhibits a continuous and linear trend, without any evident thermo-elastic anomaly, with thermal expansion coefficients αV = m>Vm>−1·∂m>Vm>/∂m>Tm>= 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.
