Structural Study Of Pyromorphite And Mimetite At High Temperature Or High Pressure

Both pyromorphite and mimetite have a general chemical formula A₅(BO₄)₃Z and are apatite isomorphism minerals with similar chemical compositions and structure(P6₃/m).Comparing with the mineralogy and high temperature high pressure structure of pyromorphite and mimetite,the author analyse the effect of the substitution of anion ions on the structure of apatite.In this dissertation some methods such as X-ray Fluorescence,X-ray Diffraction and Infrared Spectrum technics were used to analyse and compare the basic mineralogical characteristics of pyromorphite and mimetite.The refinement result of General Structure Analysis System(GSAS)indicate that the cell parameters of pyromorphite are a=9.981(?),c=7.340(?),and a=10.2393(?),c=7.4315(?)with mimetite.The replacement of P ions by the bigger As,leads to an increase of a of mimetite?2.58%,c?2.58%and V?1.25%.The infrared absorption spectra of pyromorphite are mainly absorption peak of[PO₄]^3-,while both absorption peak of[AsO₄]^3-and[PO₄]^3-appeared in mimetite.In-situ high pressure single-crystal X-ray diffraction experiments of both samples were carried out at room temperature using diamond anvil cell with synchrotron radiation source.No phase transitions of pyromorphite and mimeitie were observed up to the highest pressure 11.5 GPa.As pressure increased,the cell parameters reduced regularly,and the compression ratio was fitted by the third-order Birch-Murnaghan equation of state in Eos FIT52 software.The fitting results are K₀(pyromorphite)=49.9(4)GPa?K₀(mimetite)=51.7(2)GPa,?_a:?_c(pyromorphite)=1:0.93,and?_a:?_c(mimetite)=1:1.00.Within the experimental pressure,pyromorphite has a higher c axial compressibility than a axis,while there is nearly no difference between a and c in mimetite.These phenomena likely due to the less dense structure along a axis than along c axis.When P ion is replaced by As,the distortion along a axis is larger,leading to almost the same axial compressibility in mimetite.The high temperature mid-infra-red spectrum was used to analyse the structural stability of mimetite.As the temperature increases from room temperature to 1100?,[AsO₄]^3-stretching vibration absorption band keeps existing around 832 cm^-1,but moves to a low frequency with the increasing temperature.However,some absorption bands become weaken,even disappear.