Effects Of Environmental Factors On The Formation Conditions Of Coesite Under High Pressure And High Temperature

Silicon dioxide (SiO₂) is one of the abundant chemical materials on earth. It extensively exists in the form of quartz minerals and silicate minerals in the crust and upper mantle. From the point of view of crystal chemistry, silicon atom has the high valence and small radius, high bond energy of Si-O and the characteristics of both covalency and ionicity, which make it occur difficultly that the phase transition of SiO₂ related to Si-O bond destruction and reconstruction. This is the reason why a variety of mutual crystals of SiO₂ coexist under different pressure and temperature, and most of them exist in the form of meta-stable state under normal temperature and pressure. Therefore, studying on the phase transition of SiO₂ and the effect of environmental factors on it is of great significance in geology, materials science and crystallography.Coesite is the first high-pressure phase of quartz, which was firstly synthesized by Cos in the lab. Coesite got to be widely concerned after Chopin and Smith found the surface coesite in metamorphic rocks in western Alps and high pressure eclogite in Norway western gneiss area in 1984, respectively. In order to explain the existence of coesite in high-pressure metamorphic belts, subduction-exhumation hypothesis is proposed, of which it is considered that coesite was formed in 90 km or deeper upper mantle, and then returns to the surface of earth. The basis of calculating the depth of plate exhumation is synthetic conditions of coesite under high pressure and high temperature (HPHT). Recent research has shown that the synthetic conditions of coesite are closely related to transit path, environmental factors and initial state of initial materials, and obviously different synthetic conditions of coesite have been gained by using the different initial materials.So far the reported synthesis of coesite in the laboratory has been done under static pressure, while the existence of local high-pressure micro-area as well as its influence on the formation of coesite was not taken into consideration. The previous study show that the material composition of the solid earth is quite complicated and not well- distributed, therefore, to study the formation of coesite inside the earth, it's necessary to take into account the heterogeneity of the material composition and the nonuniformity of its stress state in the earth instead of regarding the earth as a static even fluid system. In this work, we have done experiments to study the formation of coesite under different conditions, such as uneven pressure condition, hard condition of brittle phase with strong-bond (SiO₂-Si system), soft condition of fluid phase with weak-bond (SiO₂-C system) and mixed condition of phase with strong-soft bond (SiO₂-C-Si system), which provided new thinking for explaining the formation of coesite on the surface of the earth.In the nature, coesite and diamond are often associated minerals, it is not reported whether the content of silicon or carbon has impact on the formation conditions of coesite. In the formation process of coesite, Besides should considered the static pressure action of force, the effect of local shearing in the small-scale uniform micro-distinct produced by earthquake wave or large local stress can not be ignored. Considering the process of mechanical ball milling, both a normal and shear stress can be produced by high speed collision of the steel balls. Comparing the collision between local shear stress and mechanical ball millingthe similarity between locality and shearing produced by the collision and the high-energy mechanical milling, two means of high-energy mechanical milling and HPHT is combined, We investigated the influences of environmental factors on the formation conditions of coesite under high pressure and high temperature.Firstly,In consideration of the heterogeneity of the inner substances of the earth, 10wt% hard Fe fillings are added to and mixed evenly withα-SiO₂ milled for 20h. Then, the mixture is treated under 2.0, 2.5, 3.0 and 3.5 GPa and 973k for 40min. The XRD spectrum of the synthetic samples is shown a small amount of coesite under 2.5 GPa. There exist minerals of high hardness inside the earth. When these high hardness minerals are arranged like diamond anvil cell (DAC), local high-pressure micro-area can appear. It will be possible for small-sized coesite to form in small local high pressure micro-area, the corresponding pressure of which is 2.5 GPa.Different ratio ofα-SiO₂, and silicon powder mixtures were pretreated by high energy ball-milling and XRD spectra showed that the ball-milling method and addition of silicon were conducive to the formation of amorphousα-quartz. SiO₂–Si system samples(SiO₂ : Si=6:1,SiO₂ : Si= 3:1,SiO₂ : Si= 1:1,SiO₂ : Si=1:3,SiO₂ : Si=1:6)dealt after 10h by the high-energy mechanical milling and HPHT condition of 3.8GPa, 1273K, coesite crystals existed in that one with mass ratio of 1:6. Andα-quartz and silicon crystals appeared in the other samples. Samples with the mass ratio of 1:6 ball-milled after different time were dealt by HPHT. We found that coesite crystals did not appear in any samples (ball-milling after 5h, 20h) besides the one ball-milling after 10h. This shows that under the hard and brittle covalent environment supplied by Si-Si covalent bond-type material in unique diamond structure of silicon, it is conducive to the formation of more uneven small-scale high-pressure region and amorphization ofα–quartz. Especially if combined with the ball-milling method during which there is intermediate metastable phase, both of them are conducive to the formation of coesite. On the other hand, the surface atoms were activated by the refining of amorphous Si under high-energy mechanical milling. Due to the strong bonding adhesion between Si atoms, Si atom onα- quartz grain surface have a strong affinity for oxygen atom under sealing environment with high pressure high temperature where free oxygen is in the lack, will prevent displacement from neighbor atomic and thereby constrain the Markov-type homogeneous change from hexagonalα- quartz to monoclinic coesite. The role of these two factors became the reason that coesite formed only under the condition of 3.8GPa, 1273K and 10h milling, with the ratio ofα-quartz and silicon mixed powders 1:6.Graphite as the single phase of carbon atom, the electronegativity of which is 2.5, soft and compression rate is of 40% -52%, with the layered structure and molecular interaction between layers is of weak, easy to slide, and very small hardness which is only 1. According to these properties of graphite, we looked the SiO₂-C system (SiO₂: C = 6:1, SiO₂: C = 3:1, SiO₂: C = 1:1, SiO₂: C = 1:3, SiO₂: C = 1:6) as the soft condition of fluid phase with weak-bond. Through a series of high temperature high pressure experiments, it is found that the time of amorphous SiO₂ was prolonged due to buffer resulted from the formation of carbon layer in the sample ball milling after 60h.Under the condition of 3.8GPa and 1273K, coesite crystals were contained in all samples in addition to only one with mass ratio of 1:6 of which crystallized product isα-quartz, carbon and iron silicate. The number of diffraction peaks of coesite increased and peak intensity also grew with the decreasing carbon content in the mixture.In order to verify the relationship between the presence of carbon and the formation condition of coesite, we have dealt five samples with different ratio under different HPHT conditions for 40 min. Results showed that coesite formation did not appear in any samples below 3.0 GPa pressure. Under 3.4 GPa pressure, single phase of α-quartz appeared in all samples besides the one with ratio of SiO₂: C = 6:1 in whichα-quartz and coesite existed as mixing phase.Under 3.8GPa pressure, the sample with ratio of SiO₂: C = 6:1 transited into single-phase coesite, and those with ratio of SiO₂: C = 3:1, SiO₂: C = 1:1, SiO₂: C = 1:3 transited intoα-quartz crystal. As for the sample with ratio of SiO₂: C = 1:6 single phaseα-quartz appeared in it. It can be concluded that within the range of 3.0-3.8GPa pressure, the less carbon content, the lower pressure for the formation of coesite: When carbon content in the mixture is less than14%, synthesized condition of coesite is at 3.4GPa pressure and 3.8GPa pressure for the formation of single-phase coesite; When carbon content is more than 25% less than 75%, coesite could form up to 3.8 GPa pressure but not under 3.4GPa pressure; When carbon content is greater than 86%, coesite could not form until 3.8GPa. For the SiO₂-C system, it is not conducive to form coesite by increasing carbon content and the maximum carbon content to form coesite is 86%. This indicated that more graphite content, more soft the environment and surface pressure is consumed and has fluidity, so that the effective pressure onα-quartz decreased, delay the real, smallest formation pressure of coesite.In this study, we detected the effect of strong - weak bond mixed environment on the phase transition ofα-SiO₂ under high pressure and temperature condition. For the SiO₂-C-Si system (SiO₂: C: Si = 3:1:9, SiO₂: C: Si = 1:3:3), carbon and silicon became amorphous after milled 20h for the one of SiO₂: C: Si = 3:1:9 which was 40h shorter comparing to the SiO₂-C system of 60h. Graphite ,carbon and silicon became completely amorphous for SiO₂: C: Si = 1:3:3 mixture after milling 60h.For the case of SiO₂: C: Si = 3:1:9 and SiO₂: C: Si = 1:3:3, the coesite formation were not found after 40 min under high temperature and pressure of 3.8GPa, 1273K.Despite the uneven local small-scale area forming surface coesite is a new mechanism with great possibility. However, the synthesis of coesite closely related with environmental factors, large-scale presence of coesite has not been found in the crust until now.