Carbonates (e.g., MgCO3, CaCO3) have been proposed as potential host materials for carbon in Earth’s mantle. New high-pressure research results on the iron-containing magnesite, referred to as ferromagnesite [(Mg,Fe)CO3], which was previously considered less likely to be stable in the mantle, now reveal that the effects of the Fe2+ electronic spin transition would make it a potential carbon host in the deeper mantle. The electronic spin transition in ferromagnesite at 40 GPa (equivalent to the pressure at the top of the lower mantle) reduces the unit cell volume to a level even smaller than that of pure magnesite—likely making ferromagnesite more stable in the mantle. Together with newly observed elastic and vibrational properties, the properties of the iron/magnesium-bearing carbonate at relevant pressures and temperatures of the planet’s interior are likely very distinct from that on Earth’s surface, affecting our understanding of the deep-carbon storage.
To date, ferromagnesite is the only common deep-mantle carbonate mineral containing iron known to undergo this spin-pairing transition—unpaired high-spin electrons are forced to pair together in the low-spin state due to the extreme pressures exerted on the lattice from gravitational attraction of Earth materials. As such, the effects of the spin transition on the behavior of this deep-mantle carbonate are of particular interest. A recent study , led by Prof. Lin and co-workers at University of Texas at Austin and University of Chicago, examined the high-pressure vibrational and elastic properties of iron-bearing magnesite across the spin transition by combined optical Raman and X-ray diffraction spectroscopies in a high-pressure diamond-anvil cell. The distinctive unit cell volumes, compressional and vibrational properties between the high-spin and low-spin states at 40 GPa are all indicative of the peculiar effects of the spin transition, which were not considered previously. The observed effects of the spin transition put a new spin on our understanding of the stability and properties of potential carbon carriers in the deep mantle.
Figure: Visual representation of the unit cell of magnesian siderite. The percentage of volume collapse after the spin transition at 45 GPa is shown for the c and a axes.