Temperature-induced phase transition in phlogopite revealed by Raman spectroscopy

Raman study of a natural hydrous phlogopite was carried out at temperatures up to 500 degrees C for the first time. Evolution of four well-resolved Raman modes at wavenumbers 196, 278, 322, and 682 cm(-1) was followed in detail with temperature increase. The analysis of data reveals linear decrease of vibrational wavenumbers in the studied temperature range, with small but experimentally significant discontinuities occurring at a temperature of 365 +/- 15 degrees C. Although the overall appearance of Raman spectra remains intact on crossing this temperature, the presence of discontinuities, as well as a marked difference between Gruneisen parameters calculated for high- and low-temperature ranges, signifies the presence of a temperature-induced phase transformation. By combining and correlating the results of the present Raman study with the high-temperature X-ray work performed by Tutti et al. [High-temperature study and thermal expansion of phlogopite, Phys. Chem. Miner. 27 (2000) 599-603] we arrive at the interpretation of a temperature-induced structural phase transformation in phlogopite without a significant symmetry change, with an underlying microscopic mechanism involving deformation of Mg octahedra and rotation of tetrahedral grid from ditrigonal toward hexagonal at the transition temperature. (C) 2008 Elsevier Ltd. All rights reserved.