Elastic Systematics of the Cubic Rocksalt Structure: Pressure Derivatives and Anisotropy Across Oxides, Halides, and Sulfides

Abstract

Single-crystal elastic constants and pressure derivatives are compiled for B1 rocksalt oxides, halides, and sulfides alongside B2, B3, garnet, and spinel analogues to characterize the elastic systematics of the (Mg,Fe)O solid solution and empirically test the Anderson and Liebermann (1970) prediction that dCij/dP is controlled by crystal structure rather than composition. At ambient conditions, C11 and C44 decrease monotonically with increasing Fe content across the (Mg,Fe)O solid solution while C12 increases, driving AZ toward the FeO endmember; compression is demonstrated to be a far more efficient mechanism for reducing anisotropy than Fe substitution. All B1 rocksalt compositions, regardless of mineral class, satisfy dC11/dP > 2.5 × dC12/dP and dCs/dP > 2 × dC44/dP, forming a ratio-bounded region in dCij/dP space that no B2, B3, garnet, or spinel composition satisfies, constituting the first multi-class empirical confirmation of Anderson and Liebermann (1970). (Mg0.92Fe0.08)O undergoes an isotropic crossover (AZ = 1) at ρ ≈ 4.2 g/cm3 and reaches AVSmax ≈ 37% and AVP ≈ 16% at the highest sampled density, far exceeding Pyrope and Ringwoodite, neither of which undergoes an analogous inversion. Within the alkaline-earth oxide series, B1–B2 transition pressure increases systematically with dC44/dP, while FeO departs from this trend, consistent with d-electron bonding modifying its lattice dynamics. These results confirm that dCij/dP ratio bounds provide a structure-diagnostic tool applicable to high-pressure phases of uncertain crystallography and carry direct implications for the sign and lateral variation of seismic anisotropy in the lower mantle.