Optical anisotropy in the spinel group; a polishing effect

Examination of > 100 polished sections of magnetite, chromite, franklinite and jacobsite from approx 50 localities used two final polishing procedures. Using a 0.1 mu m diamond suspension on 'microcloth' always led to weak optical anisotropy, except for 100 and 111 sections, whereas use of an alkaline silica solution on 'microcloth' gave sections which remained isotopic with the exception of five zoned magnetites. Electron channelling pattern images with SEM showed a strongly damaged surface for samples prepared with the diamond suspension and a perfect undisturbed surface for those prepared using an alkali silica solution. This convincingly indicates that the anomalous optical anisotropy effects in the spinel group are usually caused by mechanical polishing procedures.