The process of grinding micas in an agate mortar to a powder sufficiently fine for x-ray work is both tedious and awkward. Even after a thorough grinding the grains still have a sufficient size to give spotted films because of single crystal reflections. This effect can be avoided by prolonged grinding, but then the lines will often be blurred and too poorly defined for accurate measurement, probably as a result of distortions of the crystal structure because of the weak bonding between individual layers of the mica lattice. Crushing in the several different types of commercial mills gives equally unsatisfactory results.