Anomalous thermal properties and spin crossover of ferromagnesite (Mg,Fe) <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>CO</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:math>

Ferromagnesite (Mg,Fe)CO3, also referred to as magnesiosiderite at high iron concentration, is a solid solution of magnesite (MgCO3) and siderite (FeCO3). believed enter the Earth's lower mantle via subduction considered major carbon carrier in mantle, playing key role deep cycle. Experiments have shown that ferromagnesite undergoes pressure-induced spin crossover, accompanied by volume elastic anomalies, lower-mantle pressure range. In this work, we investigate thermal properties (Mg,Fe)CO3 using first-principles calculations. We show nearly all are drastically altered including anomalous reduction volume, softening bulk modulus, increases expansion, heat capacity, Guneisen parameter. Remarkably, anomaly capacity remains prominent (up 40%) temperature without smearing out, which suggests crossover may significantly affect subducting slabs