Study of the Influence of Oxygen Fugacity on the Firing Behaviour of Phyllosilicates: Methodological Notes

Anna Lepora (anna.lepora@unifr.ch) & Bernard Grobéty (bernard.grobety@unifr.ch)

Intitut de Minéralogie et Pétrographie, Pérolles, CH-1700 Fribourg, Switzerland

We calibrated a vertical gas-mixing furnace (CO₂-H₂ mixtures) in the temperature range 600-900°C, where the actual fugacities are lower than the thermodynamically expected ones (Beckett et al., 1997). The reproducible behaviour of the furnace allows us to apply the identified correction factors to the gas ratios, instead of modifying the set-up of the kiln. In literature, the data on the interactions of phyllosilicates with the firing atmosphere are strongly inhomogeneous in the applied conditions (Sanz et al., 1983, Burkhard et al., 1995). Besides the studies on the redox reactions, phenomena of adsorption / desorption of gaseous molecules, or evolution of hydrogen and other species during dehydroxylation (Brindley et al., 1987; Heller-Kallay et al., 1989) have been hypothesised for several phyllosilicates. We have tried to take this crossing of factors into account, testing in particular whether oxygen fugacity is the only determining factor, or the concentrations of the species other than oxygen are also significant. An iron-rich 3T-phengite (Fe₂O₃=2.12%w, FeO=3.06%w) was fired between 600 and 1100°C, in conditions of fixed gas ratios, and then in conditions of fixed oxygen fugacity (log ƒO₂ range: -10 ­ -23): spectrophotometric and XRD results are presented. To dynamically monitor the breakdown of phyllosilicates, we have then tried to apply the theory of gas-mixing furnaces to a High-Temperature diffractometer, set up in order to control the atmosphere in the camera. The obtained fugacities have been tested monitoring the evolution of iron and iron oxide samples from 600 to 1100°C. Sensible improvement can be noted when a gas-mixing chamber equipped with a Pt catalyst is placed before the camera.

Beckett JR, Mendybaev RA, Geochim. Cosmochim. Acta, 61, 4331-4336, (1997).

Sanz J, Gonzàlez-Carreño, Gancedo R, Phys. Chem. Min, 9, 14-18, (1983).

Burkhard DJM, Ulmer GC, Phys. Chem. Min, 22, 507-510, (1995).

Brindley GW, Lemaitre J, Chemistry of clays and clay minerals, Mineralogical Society Monograph, 6, 319-370, (1987).

Heller-Kallai L, Miloslavsky I, Grayevsky A, Am. Min, 74, 818-820, (1989).

IR Spectroscopy of Thermal Decomposition of Ammonium Analcime

Anna Likhacheva (alih@uiggm.nsc.ru)¹ & Evgeniy Paukshtis (vera@catalysis.nsk.su)²

¹ Institute of Mineralogy & Petrography, Siberian Branch of Russian Academy of Sciences, Prosp. Ac.Koptyuga 3, 630090 Novosibirsk 90, Russia

² Boreskov Institute of Catalysis, Prosp. Ac.Lavrentiev 5, 630090 Novosibirsk 90, Russia

The ammonium-exchanged form of natural analcime Na_1.83[Al_1.88Si_4.12O₁₂]^.1.97H₂O (r.Nidym, Siberian platform) was used as a model object to study the mechanism of thermal decomposition of natural ammonium bearing aluminosilicates by IR spectroscopy.

The DTA curve of waterless ammonium analcime contains only one peak between 400°C and 700°C corresponding to deammoniation and dehydroxylation.

The decomposition of NH₄⁺-ions starts at 400°C and reaches its half-way at 500°C, according to the gradual decreasing of the NH₄⁺-ion absorbtion bands intensity in the IR spectrum of ammonium analcime. The sharp changes of the spectrum in the region of 1500-2000 cm^-1 at 550°C are probably related to the transformation and starting destruction of the analcime framework. The narrowing of the band 1440 cm^-1 of bending vibrations of the NH₄⁺-ion at 550°C evidences for its geometry to approach to the ideal tetrahedron (Chourabi, 1981). This is probably related to the changing of hydrogen bonds between the NH₄⁺-ion and the framework due to the deformation of structural channels.

Start from 550°C up to 700°C the bands 1330, 1625 cm^-1 of ammonia bonded to Lewis acid sites (Knozinger et al., 1977) are observed in the IR spectrum. Their intensity increases with temperature in parallel with decreasing of the NH₄⁺-ion bands intensity. Thus the adsorbtion of ammonia on Lewis centres is an intermediate step of deammoniation of analcime, which may be caused by several factors. Narrow channels of the analcime structure prevent fast diffusion of ammonia formed at the decomposition of NH₄⁺-ions. At T^o > 550°C the dehydroxylation seems to proceed actively giving rise to Lewis centres. These centres are known to adsorb easily the ammonia molecules to form stable complexes (Corma, 1995).

The formation of some hydroxyls at 550°C with their subsequent destruction and formation of water molecules at 650°C is fixed by corresponding bands in the region of 3600 cm^-1.

The decomposition of ammonium and the removal of ammonia and water from the sample are almost completed at 700°C.

Chourabi B & Fripiat JJ, Clays and Clay Miner., 29, 260-268, (1981).

Knozinger H, Kriltenbrink H & Ratnasamy P, J.Catal., 48, 436, (1977).

Corma A, Chem. Rev., 95, 559-614, (1995).

Low-Temperature Heat Capacities and Derived Thermodynamic Properties of Glasses in the System Na₂O-B₂O₃-SiO₂

Yannick Linard (linard@ipgp.jussieu.fr)^1,2, Isao Yamashita³, Tooru Atake³ & Pascal Richet (richet@ipgp.jussieu.fr)¹

¹ Laboratoire de Physique des Géomatériaux - IPGP - ESA-CNRS 7046, 4 Place Jussieu, 75252 Paris Cedex 05,

² Laboratoire d'Etude de l'Altérabilité des Matériaux - SCD/DCC - CEA Valrhô, BP 171, 30207 Bagnols/Cèze cedex, France

³ Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8503 Japan, Japan

From laboratory glassware to nuclear waste disposal the wide range of applications of borosilicates is well established. Yet, comparatively few studies have been done on the thermochemical properties of this system, and, more particularly, on the low-temperature properties. However, these measurements are badly needed to determine entropies of melts, and especially to understand the relative importance of the contributions to the entropy stemming from thermal vibrations and temperature-induced structural changes. We have thus measured the low-temperature heat capacities of five silica-rich sodium borosilicate glasses from 15 K to room temperature with an adiabatic calorimeter. From these results we have calculated the partial molar heat capacities of boron and sodium oxides. The results show marked deviations of Cp from an ideal-solution model with oxide components in the system Na₂O-B₂O₃-SiO₂. This nonideality may reflect differences in boron coordination between pure vitreous B₂O₃, where B is essentially three coordinated by oxygen atoms, and the investigated glasses where B is distributed between BO₄ and BO₃ species. On the other hand, as will also be discussed, these deviations could be associated with peculiar interactions between B and Si. This will allow us to discuss the relationship between the entropy and the structure of borosilicate glasses in terms of short- and medium range orders.