Optical anisotropy in the spinel group: a polishing effect

Abstract

More than 100 polished sections of magnetite, chromite, franklinite and jacobsite from about 50 localities were examined by ore microscopic methods. During sample preparation, two final polishing procedures were applied: (a) a 0.1 μm diamond suspension on «Microcloth», (b) an alkaline silica solution on «Microcloth». With the exception of {100} and {111} sections, procedure (a) always led to weak optical anisotropy, while with procedure (b), with the exception of five zoned magnetites, sections remained isotropic. Electron channeling pattern (ECP) images on a scanning electron microscope (SEM) showed a strongly damaged surface for a), and a perfect, undisturbed surface structure for b). This convincingly indicates that anomalous optical anisotropy effects in the spinel group are usually caused by surface deformation caused by mechanical polishing procedures. The relation-ship between surface deformation and face symmetry leads to the isotropic appearance of deformed {100} and {111} sections and to the anisotropic appearance of low-symmetric {hkl} sections