Studies On Stability Of Calcite Structural Minerals Under High Temperature

With the emergence of global warming and other issues,geologists and gemologists have gradually paid attention to the carbon cycle and geological process in the deep earth.The deep carbon cycle of the earth is related to the formation of eclogite-type diamond,and the deep carbon is likely to occur in the mantle in the form of diamond,so it is of great gemological value to study the deep carbon cycle of the earth.Among them,the stability of carbonate minerals under different temperature conditions has a very important influence on carbon bringing in,carbon taking out,the existing forms and migration of carbon in the earth.The research work mainly focuses on calcite carbonate minerals such as dolomite(MgCa(CO₃)₂),manganese dolomite(Mn Ca(CO₃)₂),siderite(Fe₂(CO₃)₃)and calcite(CaCO₃).the stability of these minerals under different temperature conditions is studied by using low-temperature XRD,high-temperature Raman and high-temperature infrared experimental techniques.the main research results are as follows:At first,three carbonate minerals,dolomite(MgCa(CO₃)₂),manganese dolomite(Mn Ca(CO₃)₂)and siderite(Fe₂(CO₃)₃),were tested by low-and medium-temperature XRD,and the correlation between their cell parameters and crystal structure was obtained at different temperatures-100?-330?(173-603K).The volumetric thermal expansion coefficient and axial thermal expansion coefficient of the three minerals were obtained by fitting the cell parameters at different temperatures through the thermal equation of state.It is found that the three minerals have positive expansibility,that is,with the increase of temperature,the unit cell parameters of minerals increase.Calcite(CaCO₃)is a widely distributed mineral,which is usually intact and has various forms.It is mainly in the form of parallelly developed columns,parallelly developed plates,rhomboids or complex trigonal rhomboids in various states.The color of calcite varies with the impurities contained in it,such as light yellow,light red,brown black,etc.when it contains iron and manganese,but it is generally white or colorless,which has good ornamental value.In this paper,the behavior of calcite at high temperature of 25?-1175?(298K-1348K)is explored by using high temperature Raman.It is found that the frequency shift of Raman peaks at 1435cm^-1shows abnormal behavior at 725?,and the frequency shifts of Raman peaks at 155,286,1085 and 1435cm-1 show abnormal behavior at 775?At the same time,the frequency shift and full width at half maximum of 286,1085 and 1748cm^-1occur abnormal behaviors near 975?,which may be related to the ordered-disordered phase transition of calcite near this temperature,that is CaCO₃-?transformed into CaCO₃-?.The behavior of calcite at 15?-1000?(288K-1273K)was studied by high temperature infrared technology.It was found that the frequency shift of infrared peak at 890cm-1 occurred abnormal behavior at 775?.The infrared peak intensity at 713,890,1088,1450,1806 and 2500cm^-1is abnormal in the range of 700?800?.It is speculated that it is related to the phase transition from R-3c(CaCO₃-?)to R-3C(CaCO₃-?).