Microhardness variations in a naturally deformed grain of ilmenite

A grain of ilmenite enclosed in deformed quartz‐mica‐staurolite schist from Ducktown, Tennessee, shows a variety of optical features produced by natural deformation, recovery and recrystallisation processes. These features are consistent with dislocation processes similar to those observed in metals and other minerals, with the principal deformation modes apparently being slip on (0001) and twinning on (1011). Recrystallisation may have proceeded by sub‐grain rotation. Strain hardening associated with late‐stage deformation twinning has resulted in considerable variation in hardness, with measured Vickers Hardnesses ranging from 362 to 788. This range is substantially greater than those reported to date in standard compilations of ore microscopic data, even though detailed microprobe analysis has shown it to contain no component due to chemical inhomogeneity, and relatively little can be due to structural anisotropy. If other minerals show equally substantial effects of natural work‐hardening and annealing, the diagnostic value of microhardness determinations will be considerably reduced.