The structure and origin of Fe-bearing platelets in metamorphic rutile

High-resolution transmission electron microscopy has been used to characterize the defect microstructure of rutile from chlorite-, chloritoid-, garnet-, and staurolite-grade metapelites. Analytical electron microscopy (AEM) revealed that the rutile contained between 0.5 and 3 wt.% FeO and that the Fe content generally increased with metamorphic grade. High-resolution images, nanoprobe-AEM analyses, and electron diffraction patterns indicated that the Fe is contained within platelets generally less than 1 nm wide that have the hematite structure. The regularly spaced platelets are coherently intergrown parallel to (100) and (101) of rutile. They closely resemble platelets reported from experimental studies of Ti ³⁺- and Fe-bearing rutile. It is proposed that the platelets in natural rutile are hematite and that apparent tripling of the {010} spacings results from dynamical diffraction. The hematite (or Ti₂O₃) structure represents an end-member arrangement of pairs of face-sharing octahedra. At higher temperatures in synthetic rutile these pairs are ordered to form crystallographic shear planes. The hematite platelets are interpreted to have formed by a precipitation mechanism. This origin is consistent with the defect distribution and orientation and is supported by related experimental studies. -Authors