Effect Of Fe-Al Substitution On Crystal Structure And Water Stability Of Epidote Group Minerals

Epidote group minerals are common hydrous nesosilicate hydrous silicate minerals in plate subduction zone,which might be the main carriers in deep water cycling.The structure and water-bearing stability and even water cycling mechanism in hydrothermal system had been discussed by many scholars under deep earth conditions.The epidote group minerals frequently appear with Fe/Al substitution,for the previous studies,there are few papers on the effect of Fe-Al substitution on the structure and water-bearing stability of epidote group minerals under high temperature or high pressure.This study selected three samples of epidote group minerals(including synthetic starting material)with different Al-Fe substitution content.By applying electron probe analysis,variable temperature single crystal X-ray diffraction experiment,and variable temperature single crystal laser Raman experiment,variable temperature powder X-ray diffraction experiment,thermogravimetry experiment and high temperature and high pressure laser Raman experiment,researches and discusses of structure and water stability for epidote and zoisite with variable iron and aluminum content under high temperature and high pressure conditions.And it will provides some important theoretical bases for further research on the Earth's water cycle.We obtain following results via above analyses:1.Perform variable temperature and atmospheric pressure single crystal X-ray diffraction experiments(100K-500K)on zoisite(XFe0),zoisite(XFe0.1)and epidote(XFe0.68),the results show that:?The unit cell parameters of epidote group minerals show a non-linear increase during the heating process,and their volume thermal expansion coefficients are 3.63×10-5/K-1,2.76×10-5/K-1,2.11×10-5/K-1.?The increase in iron content in the epidote family minerals and the temperature increase make the O1-Al3-O2 plane in the M3 octahedron extend,and the O4-Al3-O8 bond angle decreases.?O7-O9-O8 bond angle is increase with higher iron content during the heating process;Si2O7 group with high iron content epidote(XFe0.68)rotates along the direction of decreasing O7-O9-O8 bond angle,while zoisite with low iron content zoisite(XFe0.1)and zoisite(XFe0)rotate to the opposite direction with the sample epidote(XFe0.68).2.The thermogravimetric experiments show that the dehydration reaction of epidote(XFe0.68)occurs at 1170k,and for zoisite(XFe0.1)the temperature is 1198k.3.By dissect temperature-variable powder X-ray diffraction data,the results showed that decomposition of epidote(XFe0.68)occurs around 1323-1373K,and the decomposition phase mostly is anorthite.On the zoisite(XFe0.1)side,the breakdown will not occur below the temperature of 1473K.4.High temperature high pressure laser Raman experiments were carried out on zoisite(XFe0.1)by external heated diamond anvil cell combined with in-situ laser Raman experimental device.The results show some clues that phase transition of zoisite(XFe0.1)occurs at 1130K/6.88GPa,and I suspected the phase transition is reversible.In a word,under the condition of high temperature and atmospheric pressure,the higher the content of Fe-Al replacement in epidote group minerals,the lower the thermal expansion of their volume.Moreover,the water stability and crystal structure stability of epidote with high iron content epidote(XFe0.68)are less than those of zoisite with less Fe-Al replacement zoisite(XFe0.1).Under the condition of high temperature and high pressure,zoisite(XFe0.1)has a phase transition reaction at 1130k and 6.88GPa.