Planar Defects in Bixbyite, A Prominent Diffusion Path

Johannes Peter, Stefan Lauterbach, Hans-Joachim Kleebe

Bixbyite is a rather uncommon manganese-iron oxide (Mn,Fe)2O3, crystallizing in the Ia3 space group, forming black cubic crystals with metallic lustre that are commonly truncated by small icositetrahedral faces at corners. Bixbyite normally occurs in small crystals reaching some mm in size. However, there is one location at Thomas Range (Utah), where large crystals up to 4 cm are found. These crystals occur in cavities of a rhyolite host rock associated with topaz, pseudobrookite, braunite, hematite, hausmannite and quartz. In contrast to the small isomorphic crystals, most of the larger bixbyite crystals from Thomas Range show distinct re-entrant facets at halfway of every edge of the cube, linked by a band of parallel linear features, crossing at the center of each cube face. These linear features were first characterized as twin boundaries, but they cannot be a result of local twinning, since any twinning operation on {100} planes of the centrosymmetric bixbyite structure would produce an identical single crystal. High-resolution transmission electron microscopy (HRTEM) was used to study the structure of those linear features. HRTEM images showed fault planes running along {100} planes of the bixbyite structure. The interfaces are atomically sharp and planar over large areas of the crystal. TEM/EDS analyses revealed an enrichment of Si in the fault regions with a simultaneous increase in Mn. The composition of the planar defects closely corresponds to the manganese silicate braunite, Mn2+Mn3+6SiO12. Convergent Beam Electron Diffraction (CBED) revealed an epitactic intergrowth of the Braunit and the Bixbyite host crystal, with the c-axis of the tetragonal mineral oriented perpendicular to the lamellae. Small rectangular shaped preticipates can be observed in contact to these lamellae, with their crystal faces running parallel to them. EDS analyses suggest that they are of a similar composition like the host crystal, with Mn, Fe and O as main components. Despite this they show contrast in TEM imaging, while CBED patterns indicate that they have cubic symmetry and are lacking a preferential orientation relationship to the host crystal. HRSTEM imaging revealed their atomic structure which differs significantly from that of the host crystal while corresponding closely to the iron manganese spinel Jacobsite. By comparing lattice simulations with the atomic structure of these preticipates, the occurrence of spinel in Bixbyite has been verified.

Left: Fourier-filtered HAADF image of a Jacobsite preticipate looking along [110].

Right: Magnified area of the left image. The black box shows a simulated view of the spinel-lattice along its [110]-zone axis.