Crystal structural features of the olivine → spinel transition

Comparisons of structural features of olivine (α phase), spinel (γ phase), and the modified spinel (β phase) lead to predictions of possible mechanisms for the olivine → spinel transitions. In the olivine structure, rigid tetrahedral edges and shared octahedral edges form columns of corner-sharing trigonal dipyramids parallel to the a axis. These rigid columns are separated by weaker, unshared octahedral edges which may be stretched to reduce cation-cation repulsion. As a result, olivine has a relatively loose structure and is stable at low pressure. At elevated pressure, olivine transforms to the more compact spinel structure, in which the rigid tetrahedral edges and shared octahedral edges form a three dimensional network instead of aligned columns. These structural differences explain how compressibility and thermal expansion may be taken up mainly by octahedral sites in olivine, but are evenly distributed over both octahedral and tetrahedral sites in spinel. Because the closest packings of oxygens and interstitial cation distributions differ between olivine (h.c.p.) and spinel (c.c.p.), the olivine structure may have to disintegrate during its transformation to spinel, so that the olivine → spinel transition involves processes of nucleation and growth. The migration of atoms across the olivine-spinel interface is thus a complicated process of random walk without a definite path. In the β phase → spinel transition, however, the diffusion of cations may follow a definite path in restricted regions because oxygen closest packings and cation distributions are similar in the two structures. If the oxygen packing remains intact during the β → γ transition, the transformation will be an intracrystalline process leading to domain structure in the spinel product.