A methodology for characterising grinding kinetics is developed based on a new criterion,'the residual fraction' to represent the performance of a grinding process. This is applied to pseudo-batch grinding of hydrargillite in a fluidised bed opposed jet mill Alpine 100 AFG. It appears to give an adequate representation and a simple mathematical expression for the change in particle size distribution with time. Approximate values of breakage and selection matrices are then calculated on the basis of previous studies, showing that the breakage function is not normalisable over the whole range of particle size. These results are used to propose a fragmentation mechanism for hydrargillite which includes cleavage of the larger particles and destructive breakage of the fine particles.