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

Abstract

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-T experiments). The reflection conditions and the structure refinements prove that the crystal of thomsonite here investigated is orthorhombic with a = 13.0809(3), b = 13.0597 (3), c = 6.6051(1) Å, V = 1128.37 (14) Å 3 , and space group Pbmn, which differs from thomsonite from different localities reported in previous studies (with a ~ 13.1, b ~ 13.06, c ~ 13.2 Å, 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 (~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 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 α V = V -1 ⋅∂V/∂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.

The low-temperature behaviour of cancrinite:an in situ single-crystal X-ray diffraction study

“…Similar findings have been reported for other openframework materials investigated at low-T and high-P (e.g. fibrous zeolites; Gatta, 2005;Gatta et al, 2004Gatta et al, , 2010.…”

A multi-methodological study of the (K,Ca)-variety of the zeolite merlinoite

“…The Raman spectrum of merlinoite is dominated by the doublet between 400 and 500 cm −1 (Fig. 3), assigned tentatively to the motion of oxygen atoms in the plane bisecting the T-O-T bond (T-O-T bending; Dutta and Del Barco, 1985;Dutta and Puri, 1987;Wopenka et al, 1998;Mozgawa, 2001;Gatta et al, 2010). Mozgawa (2001) ascribed the Raman-active bands between 400 and 600 cm −1 to the 'breathing' vibration of 4-membered rings of tetrahedra in the structure.…”

“…A very weak absorption at 320 cm −1 is barely visible. The higher frequency peak at 1087 cm −1 can be assigned to tetrahedral T-O-T symmetric stretching, whereas the 422 and 496 cm −1 bands can be assigned to T-O-T symmetric bending (Dutta and Del Barco, 1985;Dutta and Puri, 1987;Wopenka et al, 1998;Mozgawa, 2001;Gatta et al, 2010).…”

Mg-rich thomsonite-Ca from chromitites of the Lekkhoyla occurrence, Voykar-Syn’ya massif, Polar Urals

“…Спектры содержат линии 118.5, 161.3, 180.8, 219.9, 291.3, 306.0, 344.8, 390.0, 421.5, 442.2, 474.2, 494.5, 537.2, 576.4, 990.5 и 1071.1 см -1 . Позиция рамановских линий в изученном томсоните-Ca не отличается более, чем на 2 см -1 от линий томсонита-Ca (Gatta et al, 2010). В рамановских спектрах Mgсодержащего томсонита-Ca и ассоциирующего с ним томсонита-Ca из гидротермального прожилка (рис.…”