Shock Wave Equation Of State And The Stability Of High-pressure Phase Of Enstatite

Shock wave compression technique has play an important role in the understanding of the material composition thermodynamic state and physical properties of the Earth's ulterior. In this article, the stabling of high-pressure phase of one of the main candidate materials of the Earth's lower mantle ?enstatite (Mg0.92,Fe0.08)SiO3 were investigated through the experimented measurement of Hugoniot equation of state, and together with thermodynamic calculation and analysis. This is the key problem for the constraint on the possible composition of lower mantle.The main work and achierement are as following:(1) By using shock impedance matching technique and electric probe method, 5 shots of impact experiments were conducted to measure the Hugoniot equation of state for enstatite (Mg0.92,Fe0.08)SiO3 with the average initial density 3.05g/m3 from shock pressure 50 GPa to 115GPa, using two stage light gas gun. The linear relationship between shock wave velocity D and particle velocity u of our samples were obtained:D=3.701+1.527uThere the unit of relocity is km/s. No phase transition shows for enstatite with perovskite structure according to the experimental linear D-u line upto 110Gpa.(2) Because of different initial density of samples were always used in shock wave experimental measurement by different researchers, the Hugoniot data always shows contradiction sometimes. This is inconvenience for the advanced analysis and application of Hugoniot data. So, it is very necessary and important to modify the Hugoniot data for porosity.A new material parameter, where ρis density and subscript 0 and 00 represent different initial density, and PH and PH represent Hugoniot pressure of ρ0 and ρ00 which compressedto the same density ρ, was find out to keep in constant along Hugoniot. For different materials, the value of β is different. For metal βmetal=1.217ρ0-0.884, where ρ 0 is the no-porous density. By using β, Hugoniot data of different initial density samples can be simply convertedby: . The limitation of this empirical material constant β is discussed.(3) By using the new method put out in (2), Hugoniot data of no-porous samples of enstatite (Mg0.92,Fe0.08)SiO3 which initial density is 3.273g/m3, were reduced and the modified Hugoniot data shows very small dispersivity. The relationship between Shock wave velocity and particle velocity of no-porous enstatite can be expressed as:D=3.771+1.516u Where the unit of velocity is km/s. There is no evidence to shows phasetransition of the high-pressure phase of enstatite between 50-140GPa.(4) The Hugoniot of the mixture of MgO(Mw) and SiO2(St) were calculated by the additire principle of Hugoniot for mixture, and compared with the experimental Hugoniot of enstatite.The results shows that there exists large difference between both D-u relationship and P-n curve for the two Hugoniot. This means that the phase which being measured between 50 to 140Gpa of enstatite is not likely to be the mixture of MgO(Mw) and SiO2(St). So it is impossible for enstatite to decompose to oxides between 50 to 140GPa.(5) The bulk modulus K0S and its first derivative of pressure K0S' were calculated from the experimental Hugoniot data, by using Eularian limit strain theory. Our value of K0s=266GPa and K0s'=4.05 are very consistant with the value of static high pressure experiments. Considering that no phase transition of enstatite with perovslite structure were found during these static high pressure experiments, the consistence of the bulk modulus also supports that the high pressure phase of perovskite structure of enstatite is stability between 50-140GPa.