Majorite: Vibrational and compressional properties of a high‐pressure phase

Majorite, the garnet‐structured high‐pressure phase of pyroxene, has been characterized by infrared (IR) spectroscopy, and X ray diffraction between 0 and 8 GPa (80 kbar). In this structure, three out of four (Mg, Fe) are in eightfold coordination and one out of four Si is in sixfold coordination. The IR spectrum indicates the existence of large tetrahedral distortions, probably associated with the small cation radii (significant anion‐anion interactions). For a sample corresponding approximately to (Mg 0.79 Fe 0.21 )SiO 3 the lattice parameter and density are a = 1151.4 (±0.1) pm and ρ = 3.737 (±0.001))Mg/m 3 . Hydrostatic compression measurements yield a zero‐pressure bulk modulus and its pressure derivative of K 0 = 221 (±15) GPa and K 0 ′ ∼ 4.4 (±4.8). The composition‐dependent volume and bulk modulus in the system pyrope‐almandine‐majorite are well constrained by the present data; for majorite, these are substantially smaller and larger, respectively, than predicted by systematics for garnets. Elasticity systematics largely fail to account for the moduli of garnets, although Anderson's seismic equation of state provides a satisfactory correlation. In transforming to higher pressure phases beyond majorite (with substantially higher densities), pyroxene does not exhibit significant increases in bulk sound velocity. Transformation to majorite probably occurs within the upper mantle and further phase transitions in pyroxene may be difficult to observe seismologically.