Cover Image, Volume 97, Issue 2


False-color scanning electron microscopy image of a fracture surface in a transparent polycrystalline magnesium aluminate spinel. Areas of sub-micrometer grains (~500 nm) were observed within a matrix of larger grains (~25 μm); suggesting the presence of an impurity phase. Impurities were detected at grain boundaries and triple junctions of the smaller grains with transmission electron microscopy and these can lead to opacity in the spinel. Micrograph courtesy of Marc Rubat du Merac.