Water decreases displacive phase transition temperature in alkali feldspar

Water can be incorporated as defects and influence physical properties of feldspars. Extensive works have investigated the temperature-induced displacive phase transition in alkali feldspar and found that the transition temperature is dependent on both the orthoclase composition and degree of Al–Si order. However, there has been so far no report concerning water effects on the displacive phase transition temperature in alkali feldspars. Here, we carried out in situ high-temperature X-ray diffraction and Raman spectroscopic investigations from 20 to 800 °C on alkali feldspars with different water contents. The water content was quantified by FTIR ranging from 70 to 233 wt. ppm. The high-temperature X-ray diffraction and Raman results consistently show that the displacive phase transition occurs at 200–300 °C and 500–600 °C in the water-rich and water-poor samples, respectively. It means that water can decrease the displacive phase transition temperature. Since displacive phase transition in minerals can lead to a wide variety of elastic anomalies, this study may have implications for understanding the seismic discontinuities and heterogeneities in the Earth’s crust.