International Geologiical Congress - Oslo 2008

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MPE-01 General contributions to experimental petrology and mineralogy in honour of Professor Surendra Saxena

 

Water molecular spectra of NaCl aqueous solutions at high temperatures and pressures

 

Ronghua Zhang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Shumin Hu, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Xuetong Zhang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
 

 

Phase transitions of NaCl-H2O and their FT-IR spectra in the temperatures (T) range 25°-1100°C and pressure up to 3 GPa have been observed and examined using a new design hydrothermal diamond anvil cell(HDAC), when HDAC connected to both a light and an infrared microscope (or 10 GPa at 25°C with synchrotron XRD). There are two windows in the HDAC cell that offer visual access to the sides of the heater assemblage. The sample chamber, located between two diamonds. The visual observations of the phase relationship of NaCl solutions (29-40 wt % NaCl) start off with the 3 phase (halite+liquid+vapor, i.e. H+L+V) co-existence region, at room T and P. With continued heating, the path of phase transition follows an isochore of liquid observed along (H+L+V)-> (H+L)-> L, or H+L+V-> L, or H+L+V->L+V->L (supercritical fluid, if in the case P> Pc, w, the critical pressure of water and T>Tc, w, the critical temperature of water). Pressure could be determined by using molecular spectrum of quartz. We also used a gold particle as a pressure sensor put in the diamond cell while we applied the Synchrotron X ray diffraction method to HDAC experiment. Then pressure value obtained by using Au pressure sensor from >100°C to 500°C for the same HDAC was about 3 GPa. Experiments found O-H stretch and H-O-H bend vibration frequencies increase with increasing T and salinity. The maximum concentration of the symmetric O-H stretch was obtained in the case at 330°C, just near to the critical point of water. At 330°C, a big shape peak of stretching vibration ν1,3 was reached at 3623 cm-1, ν1,32 at 5248.67 cm-1 , while the H-O-H bend at 1600 cm-1 weakens. Before reaching 330°C, from 29 °to 300°C a wide band from 3200 to 3700 cm-1 reflects a deconvoluted spectrum of O-H stretching vibrations corresponding to the hydrogen bond and the non-hydrogen bond, and accompanying the H-O-H bend vibration. The O-H stretch vibration dominates and the breakdown of the hydrogen-bonding network occurs as close to and above Tc, w. The main vibration bands of the spectra of a liquid phase of NaCl-H2O as >400°C were found to split, e.g. at 502 °C O-H stretching vibrations fall at 3659± cm-1 and 3265.5± cm-1 respectively, which indicates two kinds of water molecules occur in one liquid phase. One is strong hydrates around Cl-bearing species and clusters; the other is simple water molecule in resulting of two kinds of water molecules in one phase. The breakdown of hydrogen bonding inter-molecules results in lowering dielectric constant of water. Each anion reacts with its neighboring water molecules, which affects OH vibration bands with characteristic peak frequencies and bandwidths. OH stretch and H-O-H bend vibration frequencies of water from NaCl-H2O increase with salinity at T from 25° to 300°C, 3-10GPa. But their vibration frequencies ν13 coupling with bend vibration decrease from 5300 to 4800 cm-1 and 2ν13 from 7000 to 6800 cm-1 as pressure increase from 1 to 10 GPa at 25°C.

 

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