Summary: Cenomanian glauconite grains from the Isle of Wight often possess a thin, pale green shell or ‘corona’ of glauconite. Optical investigation led to recognition of two types of microstructure in these shells; an outer layer with tangentially orientated c-axes, and an inner isotropic layer. A third type of layer with radially orientated c-axes occurs both subjacent to the corona and at lower levels in glauconite grains. This latter occurrence is frequently a relic of a former corona, and indicates two or more periods of glauconite formation. The disposition of the microstructures allows some conclusions to be drawn with respect to the depositional history of individual glauconite grains.