The temperature behaviour of water in leucite

Naturally leucite crystallizes in the cubic phase, with space group Ia3d (Peacor, 1968). On cooling below T = 625°C it undergoes a phase transition to a tetragonal I4/a form (Mazzi et al. 1976); there are indications, however, that an additional tetragonal phase is stable over a narrow temperature interval (Lange et al. 1986). Palmer et al. (1997) have shown that the displacive phase transition to tetragonal symmetry is due to twisting of tetragonal prisms of corner-linked (Al,Si)O4 tetrahedra about [001] and a collapse of the [111] structural channels with concomitant volume reduction. Although nominally anhydrous (NAM), leucite typically contains significant amounts of water; this feature was reported for samples from Roccamonfina (Balassone et al., 2006) and the Alban Hills volcano (Della Ventura et al., 2008). Della Ventura et al. (2008) have shown in addition that H2O may be significantly zoned, thus providing a potential tool to monitor the evolution of the magmatic conditions with time. More recently, Martucci et al. (2011) studied the dehydration of synthetic B-substituted leucite (KBSi2O6) by synchrotron powder diffraction and concluded that the structural modifications accompanying the tetragonal cubic transition is associated with the migration of H2O molecules through the [111] channels. We relate here a single-crystal high-T in situ FTIR study of a set of natural inclusion-free leucite phenocrystals occurring within lava flows, pyroclastic rocks or ejecta in the Roman Comagmatic Province. The spectra show broad absorptions in the 4000-3000 cm-1 region consisting of overlapping components around 3604, 3500 and 3250 cm-1. Interestingly, two different types of spectra are observed in the H2O stretching region, indicating that water molecules may be trapped in leucite in two different environments (hereafter “type I” and “type II”). These different H2O types are systematically associated with samples from different volcanic areas, thus suggesting a possible role of the petrological conditions (pressure, temperature) of crystallization on the H2O entrapment in leucite. FTIR-FPA images show significant H2O zoning across the samples; crystals with homogeneously-distributed water were selected for the dehydration experiments, done using a Linkam T600 heating stage fitted under a NicPlan FTIR microscope at University Roma Tre. The evolution of the water loss as a function of T was monitored by measuring the principal H2O water absorption. The data indicate a continuous water loss with a break in the trend; in “type I” leucite the slope change occurs at ~ 500°C, and dehydration is complete at T > 600°C, probably close to the transition temperature. In “type II” leucite, the slope change occurs at ~ 350-400°C, and dehydration is complete at ~ 500°C. This behaviour is compared with isostructural materials like analcime or pollucite. Mazzi, F., Galli, E., and Gottardi, G. (1976) Am. Mineral., 61, 108-115. Peacor, D.R (1968) Z. Kristall., 127, 213-224. D.C. Palmer, M.T. Dove, R.M. Ibberson, B.M. Powell (1997) Am. Mineral. 82, 16-29. G. Balassone, A. Beran, G. Fameli, C. Amalfitano, C. Petti (2006) N. Jahr. Miner. Abh., 182, 149-156 G. Della Ventura, F. Bellatreccia, M. Piccinini (2008) Am. Mineral., 93 1538-1544. Lange, R.A., Carmichael, LS.E., and Stebbins, IF. (1986) Am. Mineral., 71, 937-945