COMPARATIVE COMPRESSIBILITIES OF SILICATE SPINELS - ANOMALOUS BEHAVIOR OF (MG,FE)2SIO4

Compressibilities of five silicate spinels, including gamma-Mg2SiO4, gamma-Fe2SiO4, Ni2SiO4, and two ferromagnesian compositions, were determined on crystals positioned in the same high-pressure mount. Subjection of all crystals simultaneously to the same pressure revealed differences in compressibility that resulted from compositional differences. Ferromagnesian silicate spinels showed an anomalous 13 percent increase in bulk modulus with increasing iron content, from Mg2SiO4 (184 gigapascals) to Fe2SiO4 (207 gigapascals). This result suggests that ferrous iron and magnesium, which behave similarly under crustal conditions, are chemically more distinct at high pressures characteristic of the transition zone and lower mantle.