Natural smithsonite (ZnCO₃) has been investigated by synchrotron radiation X-ray diffraction and X-ray absorption spectroscopy combined with diamond anvil cells up to 50 GPa at room temperature. At ambient pressure, smithsonite adopts as calcite phase with a=4.6557(11) Å and c=15.072(12) Å. Both experimental results demonstrate that no phase transition occurs in the present examined pressure range. Isothermal pressure–volume relationship of smithsonite can be well described by the third-order Birch–Murnaghan equation of state with V₀=284.6(7) ų, B₀=126.8(6) GPa and (fixed). The axial compressibility of smithsonite presents clear anisotropy with α_c=3.60×10⁻³ Å GPa⁻¹>α_a=1.21×10⁻³Å GPa ⁻¹. ZnCO₃-smithsonite is proposed to approach the NaCl rocksalt structure on compression.

Superdeep diamonds contain unique information from the sublithospheric regions of Earth's interior. Recent studies suggest that reaction between subducted carbonate and iron metal in the mantle plays an important role in the production of superdeep diamonds. It is unknown if this reaction is kinetically feasible in cold slabs subducted into the deep mantle. Here we present experimental data on real‐time tracking of the magnesite‐iron reaction at high pressures and high temperatures to demonstrate the production of diamond at the surface conditions of cold slabs in the transition zone and lower mantle. Our data reveal that the diamond production rate has a positive temperature dependence and a negative pressure dependence, and along a slab geotherm it decreases by a factor of three at pressures from 14.4 to 18.4 GPa. This rate reduction provides an explanation for the rarity of superdeep diamonds from the interior of the mantle transition zone.

The presence of light elements in the metallic cores of the Earth, the Moon, and other rocky planetary bodies has been widely proposed. Carbon is among the top candidates in light of its high cosmic abundance, siderophile nature, and ubiquity in iron meteorites. It is, however, still controversial whether carbon‐rich core compositional models can account for the seismic velocity observations within the Earth and lunar cores. Here, we report the density and elasticity of Fe₉₀Ni₁₀‐3 wt.% C and Fe₉₀Ni₁₀‐5 wt.% C liquid alloys using synchrotron‐based X‐ray absorption experiments and first‐principles molecular dynamics simulations. Our results show that alloying of 3 wt.% and 5 wt.% C lowers the density of Fe₉₀Ni₁₀ liquid by ∼2.9–3.1% at 2 GPa, and ∼3.4–3.6% at 9 GPa. More intriguingly, our experiments and simulations both demonstrate that the bulk moduli of the Fe‐Ni‐C liquids are similar to or slightly higher than those of Fe‐Ni liquids. Thus, the calculated compressional velocities (v_p) of Fe‐Ni‐C liquids are higher than that of pure Fe‐Ni alloy, promoting carbon as a possible candidate to explain the elevated v_p in the Earth's outer core. However, the values and slopes of both density and v_p of the studied two Fe‐Ni‐C liquids do not match the outer core seismic models, suggesting that carbon may not be the sole principal light element in Earth's outer core. The high v_p of Fe‐Ni‐C liquids does not match the presumptive v_p of the lunar outer core well, indicating that carbon is less likely to be its dominant light element.