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 structure, three out of four (Mg Fe) are 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 (Mg0.79Fe0.21) SiO3 the lattice parameters and density are a = 1151.4 (¿0.1) pm and &rgr; = 3.737 (¿0.001) Mg/m3. Hydrostatic compression measurements yield a zero-pressure bulk modulus and its pressure derivative of K0 = 221(¿15) GPa amd K0'~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 of 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.