Summary: Seven samples of pelletat glauconites with varying iron and potassium contents were investigated using the electron microprobe. In-sample variation of pelletal compositions is quite important, even for samples previously separated according to grain-size, density, and magnetic susceptibility. This observation should lead to a reconsideration of previous work based upon multi-grain samples.

During the evolution of pelletal material towards micaceous glauconite compositions it is apparent that iron can be added to as wellas subtracted from the bulk pelletal composition. This transfer appears to be effected when the pellet contains a multiphase mineral assemblage as well as during the evolution of a single-phase mixed layered mineral.

A schematic phase diagram is presented which attempts to describe the phase changes as a function of K, Si, and Fe variables.