Study On The Structure And Stability Of Copper-bearing Minerals?CuFeO₂,CuFeS₂,CuS₂,CuS? Under High Pressure And High Temperature

In this thesis,diamond anvil cell?DAC?apparatus,combined with a micro laser heating system and a resistance heating device,were used to simulate the temperature and pressure environment of the deep Earth.Four copper-bearing minerals,delafossite?CuFeO₂?,chalcopyrite?CuFeS₂?,pyrite structure CuS₂,covellite?CuS?,were investigated at high pressure and high temperature by using in-situ Raman spectroscopy,synchrotron angle dispersive X-ray diffraction?ADXRD?,and scanning electron microscopy?SEM?.The first-principles theoretical calculations based on density functional theory were performed in CASTEP program to compare with experimental results.Through discussing the structure and chemical stability of the four copper-containing oxides and sulfides at high pressure and high temperature,the valence state,coordinated polyhedrons and possible models of the occurrence of copper in the deep Earth will be uncovered.?1?By heating delafossite?CuFeO₂?under high pressure,a disordered rock-salt type(Cu_0.5,Fe_0.5)O solid solution was experimentally synthesized for the first time in the range of 54 GPa and 2000 K.(Cu_0.5,Fe_0.5)O solid solution is similar to the ferropericlase(Mg_1-x,Fe_x)O,which abundantly occurs in the lower mantle.Its formation indicates that Cu²⁺ions may enter in the ferropericlase by isomorphism,which provide the possibility of the occurrence of Cu into the Earth mantle.In the rock-salt structure(Cu_0.5,Fe_0.5)O solid solution,Cu²⁺and Fe²⁺occupy same Wyckoff position and form regular octahedral coordination with the surrounding anions,indicating that the Jahn-Teller distorsion of Cu²⁺is completely suppressed.In decompression,the rock-salt type(Cu_0.5,Fe_0.5)O solid solution began to amorphize at¹4.5 GPa and was completely amorphous under ambient pressure,which is caused by the reappearance of Jahn-Teller effect of Cu²⁺at lower pressure.Fitting the Birch-Murnaghan equation of state with the observed P-V data of rock-salt structure(Cu_0.5,Fe_0.5)O yields V₀=86.6?7??³,K₀=96?5?GPa and K'=4?fixed?.?2?Chalcopyrite?CuFeS₂?was studied by systematic high-pressure and high-temperature experiments in the range of 0-30 GPa and 800-2000 K.The results of in-situ Raman spectroscopy,synchrotron X-ray diffraction,and scanning electron microscope show that chalcopyrite?CuFeS₂?will decompose to pyrite?FeS₂?and copper-rich melt phase?Cu₄FeS₃?under high pressure and high temperature conditions.The main decomposition products of chalcopyrite?CuFeS₂?are pyrite?FeS₂?instead of other copper sulfides,such as,CuS,Cu₂S,CuS₂,etc.,and the remaining Cu-containing substances exist in the form of melt,indicating that the structure and chemical stability of pyrite?FeS₂?are stronger than most copper sulfides.Its well know that the sulphophile affinity of Cu is usually stronger than that of Fe at ambient condition.However,this experimental results show that sulphophile affinity of Cu may be weaker than that of Fe under high pressure and high temperature,as well as the iron sulfides are generally more stable than copper sulfides in deep Earth conditions.?3?Pyrite structure CuS₂ was synthesized by using a diamond anvil cell?DAC?in the range of 34.1 GPa and 2000 K,which extend the known synthesis criteria of temperature and pressure.By using Raman spectroscopy and synchrotron X-ray diffraction,pyrite-type CuS₂ was found to be stable in 30 GPa without any structural phase transition.The Raman spectroscopy show that all observed Raman mode frequencies of CuS₂ increase continuously and linearly with increasing pressure.Fitting experimental pressure and volume data of synchrotron X-ray diffraction with Birch-Murnaghan equation of state gives V₀=193.8?5??³,K₀=99?2?GPa and K₀'=4?fix?.The results of first-principles calculation of CuS₂ are consistent with the experimental results.The pyrite structure CuS₂ remains stable under high pressure and high temperature,indicating that Cu²⁺may be more stable than Cu⁺in the deep Earth,and providing another potential possibility of the occurrence of Cu into the Earth mantle.?4?High pressure experiments of covellite?CuS?were carried out at room temperature and at high temperature in the range of 35 GPa and 2000 K.The results show that CuS undergoes an isostructural phase transition at⁸ GPa and then becomes amorphous at¹8 GPa.After heating at above 800 K,the two high-pressure phases of CuS will further decompose to the pyrite structure CuS₂ with a copper-rich phase.It indicates that the stability of pyrite structure CuS₂ is higher than that of CuS under high temperature and high pressure conditions.Combining the experimental results of the above four copper-bearing minerals,rock-salt type solid solution(Cu_0.5,Fe_0.5)O and pyrite structure CuS₂ can be stable under the Earth mantle conditions,providing two potential possibilities of the occurrence of Cu into the mantle.Copper ions in(Cu_0.5,Fe_0.5)O and CuS₂ are both+2 valence state,suggesting that Cu⁺may not be as stable as Cu²⁺in the deep Earth.Although both pyrite structure FeS₂ and CuS₂ have high stability under high pressure and high temperature,the chalcopyrite?CuFeS₂?decomposes to FeS₂ instead of CuS₂.It indicates that the sulphophile affinity of Fe²⁺may be stronger than Cu²⁺in the deep Earth,which is different from that at ambient conditions.This thesis provides important information of the valence state,coordination polyhedrons and the occurrence of Cu in the deep Earth.