Refinement of the α-β quartz phase boundary based on in situ Raman spectroscopy measurements in hydrothermal diamond-anvil cell and an evaluated equation of state of pure H2O

In this study, we determined the alpha-beta quartz phase transition temperatures (T(tr)s; up to similar to 924 degrees C for a total of 23 obtained under 0.1 MPa and elevated pressures) in a hydrothermal diamond-anvil cell (HDAC) at H2O pressures up to similar to 1310 MPa by using in situ Raman spectroscopy. When compared with the commonly used alpha-quartz Raman band near 465 cm(-1 )(at 24 degrees C and 0.1 MPa), the one near 128 cm(-1) is much more sensitive for detecting T-tr. The corresponding phase transition pressure (P-tr) at each measured T-tr was calculated from three equations of state (EoS) of pure H2O. The corresponding three alpha-beta quartz phase boundaries were compared with those derived from previous experimental results. The best fitted one can be represented by: P-tr (+/- 5.6 MPa) = 0.0008.T-tr(2) + 2.8056.T-tr - 1877.5, where 574 degrees C <= (tr) <= 889 degrees C with R-2 = 0.9998. Our results agree, within experimental uncertainties, with most of previous experimental data, and our newly determined alpha-beta quartz P-tr-T-tr phase boundary can be used as a reliable pressure calibrant in HDAC. Furthermore, the experimental procedures used in this study, including the use of Raman spectroscopy criterion for T-tr determination, can be easily applied to obtain improved EoS of geologically important aqueous solutions containing salt(s) at pressure-temperature conditions up to similar to 1.2 GPa-900 degrees C.