Layer silicates are generally assumed to be insulators, but electron transport may take place in nm thick particles. A combined scanning tunnelling-atomic force (STM-AFM) instrument using a conducting AFM tip has been constructed to investigate this conduction. Some layer silicates, e.g. micas (muscovite and biotite), are in fact semiconductors, conduction taking place through free electrons in the tetrahedral sheet (n-type semiconductivity) and probably through polaron hopping in the octahedral sheet. This implies that these minerals can be investigated by STM. Furthermore, micas show negative differential resistance (decreasing current with increasing voltage) at 2–5 V.