抄録
Electrostatic energies of olivine, modified spinel and spinel with M2SiO4 stoichiometry (M=Ni, Mg, Co, Fe and Mn) and those of their constituent ions were calculated by the method of Bertaut, to obtain better understanding on each polymorph; the stability relations under high pressures and the effect of ionic size on the electrostatic stability.
Stability relations of these polymorphs can be explained on the basis of proposed criteria; minerals with high repulsive energy and “simple” energetics only can survive under high pressures, where the “simple” energetics means that the electrostatic energies of constituent ions do not differ greatly from each other. Olivine does not have high repulsive energy, but involves a complex energetics. It cannot, therefore, survive under high pressures. Instability of modified spinel under very high pressures can be explained also in terms of its complex energetics. Spinel has high repulsive energy and does not involve a complex energetics. It is, therefore, stable under high pressures.
Electrostatic energies plotted against ionic radius ratio rM⁄rSi reveal the effect of ionic size on crystal energetics as follows : Electrostatic energies of M ion, O ion and the whole crystal become higher, whereas that of Si ion becomes lower with an increase of ionic radius ratio. Electrostatic energy of Mg2SiO4 olivine is slightly different from the trend of those of transition metal olivines. A rapid increase in electrostatic instability of olivine structure is observed at the ionic radius of Mn.
