Structure Of Metavariscite And Phosphosiderite Under High Pressure And High Temperature Condition

Phosphates is well known as a common accessory phase in ultrahigh-pressure rocks,which incorporates high levels of rareearth elements(REEs)and very large lithophile elements(VLLEs).Metavariscite and phosphosiderite are typical monoclinic phosphates with a pore structure.The gemmological and mineral of these samples are researched by X-ray fluorescence spectrometer,X-ray powder diffraction analysis,infrared spectroscopy and differential thermal analysis.Infrared spectroscopy results show that the samples have a[PO₄]³⁺and the functional characteristic of structural water absorption peak;X-ray powder diffraction analysis show that the samples are metavariscite and phosphosiderite;XRFS data analysis show that the main chemical composition of P₂O₅,Al₂O₃(Fe₂O₃),the major trace elements are Cr³⁺,Sc³⁺,etc.The hyperthermia powder X-ray diffracrion(XRD)is used to identify the structure of the metavariscite.The XRD patterns are analyzed in details,and which reveal that the structures of metavariscite transform with the raising temperature(upmost to 1000?).In the experimental process,the expansibility along c axis is the biggest.It transform from P2₁/n(<150?)to tridymite Al PO₄P6₃/mmc(>600?),with the intermediate temperature has metavariscite and Al PO₄(unknown)(150??500?)and tridymite Al PO₄(500??600?).The experimental results are consistent with the previous accomplishments and the heating process is irreversible.In situ high-pressure X-ray diffraction(XRD)experiments of framework-hydrous-phosphate metavariscite and phosphosiderite were carried out at room temperature using the diamond anvil cell(DAC)device with synchrotron radiation source in Shanghai Synchrotron Radiation Facility(SSRF).The maximal pressure recorded at room temperature in these experiments reached as high as 12.2 GPa.The experimental results show that the unit-cell volume is continuously decreasing with increasing pressure.For metavariscite,two phase transitions were observed in the pressure range of 5.5?7.3 GPa and 9.0?10.0 GPa.Similarly,two phase transitions also were observed in the pressure range of 4.9?7.2 GPa and 7.8?9.4 GPa of phosphosiderite.Isothermal pressure-volume relationship of metavariscite is well described by the Birch-Murnaghan equation of state.The fitted results indicate that the unit-cell volume at ambient pressure V₀ of metavariscite is 416.7?³ and the bulk modulus K₀ is 55.4 GPa by fixing its first derivative K'at 4.The unit-cell volume at ambient pressure V₀of phosphosiderite is 454.2?³and the bulk modulus K₀is 44.0GPa,the first derivative K'is 7.6.