High-temperature phase transition and local structure of a hydrous anorthoclase

The in situ Raman spectra of a hydrous anorthoclase at temperatures of 20-800 A degrees C have been measured using a LABRAM-HR spectrometer and Linkam TS 1500 heating stage. The frequencies of modes at 54, 99, 130 and 162 cm(-1) related to M-O vibrations decrease sharply and then increase drastically or keep steady at temperatures above 200 A degrees C. A knee point can be clearly seen at about 200 A degrees C for those modes. The frequency of the mode at 282 cm(-1) shows little temperature dependence. However, for the two strongest modes at 471 and 512 cm(-1), the frequencies decrease linearly with increasing temperature. From evolution of the frequencies of modes at 54, 99, 130 and 162 cm(-1) with temperature, the following conclusions can be drawn: (1) The distance of the local M-O bond shortens rather than lengthens at temperatures above 200 A degrees C; (2) The abrupt changes of the local structure of M site induce a collapse of the framework structure and displacive phase transition at 200 A degrees C; and (3) The H atoms incorporated in anorthoclase are located at the M site. These results are indicative for the structure and properties of anorthoclase at deep earth conditions.