High-Pressure Orthorhombic Ferromagnesite as a Potential Deep-Mantle Carbon Carrier

• Published in: Scientific reports Vol. 5; no. 1; p. 7640
• Main Authors: Liu, Jin, Lin, Jung-Fu, Prakapenka, Vitali B.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 06.01.2015; Nature Publishing Group
• Subjects: 147/135; 639/33/445/330; 704/2151/209; Carbon; Carbon cycle; Carbon dioxide; Carbon sequestration; Carbonates; Chemical properties; ENVIRONMENTAL SCIENCES; geochemistry; GEOSCIENCES; High pressure; Humanities and Social Sciences; Iron; Lower mantle; MATERIALS SCIENCE; mineralogy; multidisciplinary; Phase transitions; Pressure; Science; Temperature effects
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/srep07640

Knowledge of the physical and chemical properties of candidate deep-carbon carriers such as ferromagnesite [(Mg,Fe)CO 3 ] at high pressure and temperature of the deep mantle is necessary for our understanding of deep-carbon storage as well as the global carbon cycle of the planet. Previous studies have reported very different scenarios for the (Mg,Fe)CO 3 system at deep-mantle conditions including the chemical dissociation to (Mg,Fe)O+CO 2 , the occurrence of the tetrahedrally-coordinated carbonates based on CO 4 structural units and various high-pressure phase transitions. Here we have studied the phase stability and compressional behavior of (Mg,Fe)CO 3 carbonates up to relevant lower-mantle conditions of approximately 120 GPa and 2400 K. Our experimental results show that the rhombohedral siderite (Phase I) transforms to an orthorhombic phase (Phase II with Pmm 2 space group) at approximately 50 GPa and 1400 K. The structural transition is likely driven by the spin transition of iron accompanied by a volume collapse in the Fe-rich (Mg,Fe)CO 3 phases; the spin transition stabilizes the high-pressure phase II at much lower pressure conditions than its Mg-rich counterpart. It is conceivable that the low-spin ferromagnesite phase II becomes a major deep-carbon carrier at the deeper parts of the lower mantle below 1900 km in depth.