Zheng Hai-Fei

In Situ Observation of Gypsum-Anhydrite Transition at High Pressure and High Temperature

An in-situ Raman spectroscopic study of gypsum-anhydrite transition under a saturated water condition at high pressure and high temperature is performed using a hydrothermal diamond anvil cell (HDAC). The experimental results show that gypsum dissolvs in water at ambient temperature and above 496 MPa. With increasing temperature, the anhydrite (CaSO4) phase precipitates at 250–320°C in the pressure range of 1.0–1.5GPa, indicating that under a saturated water condition, both stable conditions of pressure and temperature and high levels of Ca and SO4 ion concentrations in aqueous solution are essential for the formation of anhydrite. A linear relationship between the pressure and temperature for the precipitation of anhydrite is established as P(GPa) = 0.0068T−0.7126 (250°C≤T≤320°C). Anhydrite remained stable during rapid cooling of the sample chamber, showing that the gypsum-anhydrite transition involving both dissolution and precipitation processes is irreversible at high pressure and high temperature.

Raman spectroscopic studies of the phase transitions in hexane at high pressure.

Raman spectroscopic study of n-hexane was carried out in a cubic zirconia anvil cell up to approximately 2.0 GPa. Under high pressure, the C–H stretching region of the spectrum at 2850–3000 cm−1 shows measurable changes in frequency, bandwidth, and intensity. These Raman bands shift towards higher frequencies with increasing pressure. At about 1.4 GPa, phase transition from liquid to solid was induced by compression, as was simultaneously observed with the built-in microscope.

Raman Scattering Spectroscopy of Phase Transition in n-Pentadecane under High Temperature and High Pressure

The Raman spectroscopy of n-pentadecane is investigated in a moissanite anvil cell at normal temperatures and a diamond anvil cell under pressure to about 3000MPa and at temperature from 298 to 573 K. Result indicates that at room temperature the vibration modes, assigned to the symmetric and asymmetric stretching of CH3 and CH2 stretching, shift to higher frequency and display a pressure dependent quasi-linear curve. A liquid-solid phase transition appears at a pressure of 150MPa. The high temperature solidus line of n-pentadecane follows a quadratic function of P = 0.02369T2 − 9.117T + 725.58, in agreement with previous conclusion derived from studies of other hydrocarbons. Upon phase transition, fitting the experimental data obtained in a temperature range of 283–553 K to the Clausius–Clapeyron equation allows one to define the thermodynamic parameters of n-pentadecane of dP/dT = 0.04738T − 9.117.

MEASUREMENT OF ELECTRICAL CONDUCTIVITY OF 0.001 MOL NACL SOLUTION UNDER HIGH PRESSURES

The experiments were carried out in a YJ-3000 ton press fitted with a wedge-type cubic anvil. The samples were pressed using pyropgyllite as the pressure medium. The conductance cell for the experiments was made of Teflon and platinum metal was used as electrodes. The detailed method for setting up of sample has been reported.

Raman Spectroscopic Studies of Pressure-Induced Phase Transitions on 1-Dodecene

Raman spectroscopic features of 1-dodecene are studied in a moissanite anvil cell up to 3.0 GPa at 21°C. Our data indicate that 1-dodecene is chemically stable under the experimental condition because no new Raman peaks can be observed. However, two significant discontinuities in the plots of Raman shift versus pressure indicate two phase transitions of 1-dodecene. One is liquid-solid transition at pressure of about 500 MPa, the other is solid–solid phase transition at pressure from 1300 to 1550 MPa. The latter is considered to be related to the orientational change of the plane structure of ethylene. A rudimentary phase diagrams for 1-dodecene, n-pentane, n-hexane are proposed based on the results and previous data.

Liquid Water Structure from Anomalous Density under Ambient Condition

From discussion of the structure of liquid water, we deduce that water under ambient condition is mainly composed of ice Ih-like molecular clusters and clathrate-like molecular clusters. The water molecular clusters remain in a state of chemical equilibrium (reversible clustering reactions). This structural model can be demonstrated by quantitative study on anomalous density with increasing temperature at ambient pressure.

Raman Spectroscopy of Water up to 6 kbar at 290 K

We study the Raman scattering of the stretching band from liquid water under pressure up to 6 kbar at 290 K. The result shows that the (v1)max intensity decreases with increasing pressure initially and reaches the minimum at about 2 kbar, and increases with the further increase of pressure up to about 4 kbar, then decreases again with the increasing pressure up to 6 kbar. This is in agreement with the behaviour of the average separation rOO between the nearest molecules under pressure. Additionally, the influence of pressure on water structure is also discussed.

High Pressure-Differential Thermal Analysis and Ultrasonic Wave Amplitude Analysis of Ice-Water Equilibrium at 1.5-5.0 GPa

Water is the most active component in all geological systems. It has an important effect on the physical properties of minerals and melts. It also plays a key role in the evolution of the Earth. Accurate thermodynamics data on water are currently confined to pressures below 1.0 GPa and temperatures below 900°C. Presented in this paper are new data available on theP-T properties of water at pressures up to 5.0 GPa, developed from differential thermal analysis and ultrasonic wave amplitude analysis. It has been found that there may exist another ternary point at 3.0 GPa and that ultrasonic wave amplitude change of ice-water transition shows two inflection points above 2.0 GPa, consistent with the two peaks of differential thermal curves above 2.0 GPa. It may be a new phenomenon which needs further study.

Dehydration temperature of serpentine at elevated temperatures and pressures by electrical conductivity method and its implications

Dehydration temperatures of serpentine were measured in the pressure range between 1.0 GPa and 5.0 GPa by using the electrical conductivity method simultaneously at high temperatures and high pressures. The results show that with increasing pressure the dehydration temperature of antigorite increases slightly below 2.0 GPa, but drops markedly above 2.0 GPa. This strongly suggests that high pressure would favor the dehydration of serpentine minerals and the water released thereby would be an important source of fluids involved in magmatism in a subduction zone and mantle metasomatism. Meanwhile, the greatly enhanced electric conductivity in the presence of water may be one of the reasons underlying the occurrence of a high-conductivity zone in the lower crust.