High-T FTIR study of NaNa2Mg5Si8O21(OH)3.

The extra-protonated Na Na2 Mg5 Si8 O21 (OH)3 synthetic amphibole is triclinic at room T, and undergoes a complex triclinic-to-monoclinic C¯1 ? C2/m phase transition in the T range 100-160 °C [1, 2]. Here we relate the preliminary results of a high-T FTIR study across this phase transition. Data were collected at 10 °C step interval, from room-T up to 250 °C. The room-T spectrum shows a triplet of very sharp bands, at 3740, 3727 and 3711 cm-1, respectively, which are assigned to three distinct O-H dipoles locally interacting with a strongly ordered A-cation, and a very broad band centered around 3300 cm-1. The spectra show systematic peak shift and intensity decrease with T. At 180 °C the three peaks merge into one single absorption at 3723 cm-1, and no significant change is observed beyond this T. Part of the broad band centered at 3300 cm-1 vanishes with increasing T, while a strongly asymmetric and well-defined peak at 3430 cm-1 is resolved. The intensity of this peak remains constant at any T, even for long heating stages at T > 250 °C. This absorption is assigned to an extra O-H group in the structure, which is strongly involved in hydrogen bonding with the surrounding oxygen atoms. All these changes are completely reversible during cooling. These results closely parallel those obtained by [2] with MAS-NMR spectroscopy, and constraint the phase-transition to Tc ? 180 °C. Further analysis with synchrotron-radiation FTIR at variable T is underway in the NIR region, where vibrations due to the A-O bonds occur. They should help to clarify the local environment of the OH groups. [1] Maresch, W.V., Miehe, G., Czank, M., Fuess, H. and Schreyer, W. (1991) Eur. J. Miner., 3, 899-903. [2] Liu, S., Welch, M.D., Kinowski, J. And Maresch, W.V. (1996) Eur. J. Miner., 8, 223-229.