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Order/disorder in natrolite group zeolites: A ²⁹Si and ²⁷Al MAS NMR study

¹ Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, Florida 32611-2120, U.S.A.
² Department of Chemistry, University of Manitoba, 350 Parker Building, Winnipeg, Manitoba, R3T 2N2, Canada
³ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, U.S.A.

Correspondence: ^* E-mail: neuhoff{at}ufl.edu

Disordering of Si and Al in natrolite, scolecite, mesolite, and gonnardite was investigated with ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The results indicate that with the exception of one sample of natrolite (from San Benito County, California), the natrolite, mesolite, and scolecite samples studied all exhibit small degrees (<10% Al occupancy of Si sites) of Si-Al disorder. The spectra for these samples are consistent with Al avoidance. Gonnardite is confirmed to have extensive Si-Al disorder, with only slight preferential Si occupation of the T1 site. Fits of ²⁹Si MAS NMR spectra and mathematical relations based on Al avoidance were used to calculate Si and Al occupancies across the tetrahedral sites in these minerals. Configurational entropies arising from Si-Al disorder in natrolite, mesolite, and scolecite can add an addition 1–2% [up to 11 J/(mol·K)] to the total entropies of these phases at 298.15 K, whereas it may add as much as 7% to that of gonnardite [up to 27.7 J/(mol·K)]. These results also concur with previous observations of a gap in Si-Al disordering between orthorhombic and tetragonal natrolite samples and suggest that the state of disorder in natrolite is a function of temperature. The ²⁹Si MAS NMR spectrum of gonnardite is consistent with a disordered natrolite framework structure, and not an intergrowth of thomsonite and natrolite structural domains.

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