铁的自旋转变对菱镁矿热力学性质的影响

含铁菱镁矿(Mg,Fe)CO3是碳进入地球深部的主要载体之一,铁的进入会引起矿物物理性质的变化。采用第一性原理计算方法,研究了菱镁矿含铁及铁的自旋转变对菱镁矿热力学性质的影响。含铁菱镁矿的低自旋态体积比不含铁菱镁矿小;高自旋态在低温端的体积比不含铁菱镁矿略微增大,在高温端却减小;在所研究的温压范围内,低自旋态的体积始终比高自旋态的体积小。含铁菱镁矿高自旋态的热膨胀系数减小,而自旋转变会导致热膨胀系数增加。考虑高低两种自旋态共存时的热力学性质时,计算结果表明:自旋态共存时的热膨胀系数、速度在自旋共存区间内分别呈现异常增大峰和异常减小峰,并且这些异常变化峰随着温度的升高向高压方向移动。

Effect of Spin Transition of Iron on Thermodynamic Properties of Magnesiosiderite

Magnesiosiderite(Mg,Fe)CO3]is one of the main carriers for carbon to enter the deep Earth,and the presence of iron will cause great change of mineral physical properties.The effects of ferrous iron’s spin transition on the thermodynamic properties of magnesiosiderite have been studied by first principle calculations.The volume of(Mg,Fe)CO3 low spin state(LS)decreases,while the volume of(Mg,Fe)CO3 high spin state(HS)decreases slightly at lower temperature and increases at higher temperature,as compared with that of MgCO3.In the whole range of temperature and pressure studied in this work,the volume of LS state is smaller than that of HS state.The thermal expansions of HS and LS magnesiosiderite reduce with respect to that of MgCO3,respectively.The effects of coexistence of HS and LS have been considered,in which the calculations show that the thermal expansion and velocity present abnormal increase and decrease,respectively.Meanwhile,the abnormal change peaks could move to high pressure as the increasing of temperature.