Structural Transition Of Quartz, Graphite And Silicon Under High Temperature And High Pressure

Silica is a kind of mineral that exists widely in nature. It has many kinds of polymorphs and each polymorph can transit another under different temperatures and different pressures, therefore, we can infer how the rocks near the Earth's surface form by the study of the silica near the Earth's surface. Coesite is a polymorph of silica existed in high pressure. Coesite is result of ultrahigh-pressure metamorphism. The discovery of coesite bearing rocks indicates that the rocks near the Earth's surface have been subducted by strong tectonic movement to the depth of more than 90km by plate movements and experienced ultrahigh-pressure metamorphism there. For 50 years,the study of coesite in the earth′s surface has been regarded as a"window"for knowing the earth.In labs, much work on the synthesis of coesite by a high static pressure and high temperature.,including the effect of the initial state of materials on the synthesis of coesite has been carried out.Some scientists had advanced the hypothesis of plate subduction-exhumation. Because there has been no suitable and direct method with which to test and verify this formation mechanism of coesite up to now, although the hypothesis of plate subduction-exhumation has been queried, it is still the only one explanation of coesite formation in the earth's crust. However, it must be pointed out that there are many differences between the pressure induced by dynamic collisions and high static pressure. In the former case, the pressure has both a normal and shear stress; it has obvious localization of and inhomogeneity in the pressure and the temperature, and a discontinuity in the role of time. In the latter case, the pressure is hydrostatic or quasi-hydrostatic; so it is mainly a normal stress with a more homogeneous, and continuous role of time. For the hypothesis of subductionexhumation of plate, only the high static pressure condition subducting into the upper-mantle has been considered while the collisions between the plates before and during the subducting process have been not. The role of mechanical ball milling (MBM) has been studied in our group. During the process of high-energy MBM, high local pressure and temperature can be generated by the impact of the steel ball, i.e. 3.0 ~ 6.0 GPa respectively. Also both a normal and shear stress can be produced by the high-speed collision of the steel ball with three-dimensional vibration. Comparing the collision between the plates in the earth and MBM, although the differences in space and speed of the collisions between the plates in the earth and MBM in laboratory are very great, and also all the specific ways of collision are not the same, it is found that there is a common localization features of pressure (including normal and shear stress) and temperature for the two kinds of collision phenomena. So through their similarity, these two seemingly very different in space and unrelated collision phenomena can be related to each other. MBM can be used to study a modeling synthesis for the coesite in the earth's crust according to the former common features, and can also be used to study the age-old collision dynamics of plates according to the latter differences through high frequency collision of balls in laboratory. Up to date, the effects of local dynamic pressure and shear stress on the synthesis of coesite(stishovite and diamond )have not yet been studied. So the research which is the transition ofα-Quartz—Coesite (α-Quartz—Stishovite) and Graphite—Diamond by a laboratory method combining high-energy mechanical ball milling and high static pressure is an important subject.As a kind of typical extreme condition, high-pressure Method has been developed fast recent years on the aspects of big chamber and high pressure. The properties of high-pressure method are shortening the distance between the atoms effectively, changing the states of electrons and affecting phrases changing courses. In the condition of high pressure and high temperature, the materials are given high energy. So the high pressure method is effective to synthesize new materials. High pressure and high temperature method can mimic the state in the Earth. That can enlighten and help us on comprehending many geologic phenomena.In this paper, we were selected the mixture of graphite andα-quartz, the mixture ofα-quartz, graphite and silicon as original materials. The original materials were treated with high-energy mechanical ball milling . We found that the mixture of graphite andα-quartz was completely amorphous by high-energy mechanical ball milling 120 hours, but the mixture of graphite,α-quartz and silicon has been achieved completely amorphous only by high-energy mechanical ball milling 20 hours. This shows that the material of silicon is in favor of the amorphous of original material.The various samples were treated with high temperature and high pressure, which were made by high-energy mechanical ball milling. We researched the structure transition of the samples in different conditions of high Pressure and high temperature. The mixture of graphite andα-quartz (graphite :α-quartz =2 : 3)which was made by high-energy mechanical ball milling 120 hours was treated with 2.5 GPa,873K and 3.8 GPa, 873 K .We found that synthesizedα-quartz crystal and graphite crystal in these conditions of high temperature and high pressure. It synthesizedα-quartz crystal, graphite crystal and iron silicate crystal in 2.5 GPa, 1273K. But in 3.8 GPa. 1273 K ,we not only synthesizedα-quartz crystal, graphite crystal but also synthesized coesite crystal .The mixture of graphite ,α-quartz and silicon(silicon : graphite :α-quartz = 65:14:21) which was made by high-energy mechanical ball milling 20 hours was treated with 3.8 GPa,873K . We found that synthesized coesite crystal, silicon crystal and silicon carbide crystal. It compared with two mixed that the temperature synthesis condition of coesite was reduced. The mixture (silicon : graphite :α-quartz = 65:14:21)which was made by high-energy mechanical ball milling 40 hours was treated with different conditions of high pressure and high temperature, that we found it synthesized Fe3Si. The experimental results show that the silicon is in favor of reducing the temperature of synthesis coesite and reducing the amorphous time of graphite.