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首页> 外文期刊>Crystal growth & design >Formation of Mg-Orthocarbonate through the Reaction MgCO3 + MgO = Mg2CO4 at Earth's Lower Mantle P-T Conditions
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Formation of Mg-Orthocarbonate through the Reaction MgCO3 + MgO = Mg2CO4 at Earth's Lower Mantle P-T Conditions

机译：通过反应MgCO3 + MgO = Mg2CO4在地下的较低的地幔P-T条件下形成Mg-二碳酸酯的形成

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摘要

Orthocarbonates of alkaline earth metals are the newly discovered class of compounds stabilized at high pressures. Mg-orthocarbonates are the potential carbon host phases, transferring oxidized carbon in the Earth's lower mantle up to the core-mantle boundary. Here, we demonstrate the possibility for the formation of Mg2CO4 in the lower mantle at pressures above 50 GPa by ab initio calculations. Mg2CO4 is formed by the reaction MgCO3 + MgO = Mg2CO4, proceeding only at high temperatures. At 50 GPa, the reaction starts at 2200 K. The temperature decreases with pressure and drops down to 1085 K at the pressure of the Earth's core-mantle boundary, approximately 140 GPa. Two stable structures, Mg2CO4-Pnma and Mg2CO4-P2(1)/c, were revealed using a crystal structure prediction technique. Mg2CO4-Pnma is isostructural to mineral forsterite (Mg2SiO4), while Mg2CO4-P2(1)/c is isostructural to mineral larnite (beta-Ca2SiO4). Transition pressure from Mg2CO4-Pnma to Mg2CO4-P2(1)/c is around 80 GPa. Both phases are dynamically stable on decompression down to the ambient pressure and can be preserved in the samples of natural high-pressure rocks or the products of experiments. Mg2CO4-Pnma has a melting temperature more than 16% higher than the melting temperature of magnesite (MgCO3). At 23.7, 35.5, and 52.2 GPa, Mg2CO4-Pnma melts at 2661, 2819, and 3109 K, respectively. Acoustic wave velocities V-p and V-s of Mg2CO4-Pnma are very similar to that of magnesite, while universal anisotropy of Mg2CO4-Pnma is stronger than that of magnesite, as well as the coefficient A(U) is larger for orthocarbonate. The obtained Raman spectra of Mg2CO4-Pnma would help its identification in high-pressure experiments.
机译：碱土金属的正碳酸盐是新近发现的一类在高压下稳定的化合物。镁原碳酸盐是潜在的碳主相，将地球下地幔中的氧化碳转移到核幔边界。在这里，我们通过从头计算证明了在高于50 GPa的压力下在下地幔中形成Mg2CO4的可能性。Mg2CO4由MgCO3+MgO=Mg2CO4的反应生成，仅在高温下进行。在50 GPa时，反应开始于2200 K。温度随压力降低，在地核-地幔边界的压力下下降至1085 K，约为140 GPa。用晶体结构预测技术揭示了Mg2CO4-Pnma和Mg2CO4-P2（1）/c两种稳定结构。Mg2CO4-Pnma与矿物镁橄榄石（Mg2SiO4）同构，而Mg2CO4-P2（1）/c与矿物larnite（β-Ca2SiO4）同构。从Mg2CO4-Pnma到Mg2CO4-P2（1）/c的转变压力约为80gpa。这两个阶段在减压到环境压力时动态稳定，可以保存在天然高压岩石样品或实验产品中。Mg2CO4 Pnma的熔化温度比菱镁矿（MgCO3）的熔化温度高出16%以上。在23.7、35.5和52.2 GPa下，Mg2CO4 Pnma分别在2661、2819和3109 K下熔化。Mg2CO4-Pnma的声波速度V-p和V-s与菱镁矿非常相似，而Mg2CO4-Pnma的普遍各向异性比菱镁矿强，且正碳酸盐的系数A（U）更大。获得的Mg2CO4 Pnma的拉曼光谱将有助于在高压实验中对其进行鉴定。

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