Results of positron annihilation studies in thermally treated natural pyrophyllite (PP) are presented. In this material we have found the positron lifetime component to range between 0.5 and 1 ns. This can be associated with voids whose evolution follows the dehydroxylation process induced by thermal treatment. The annealing of the PP at 1100^ˆC caused the transition of the voids to micro-pores whose estimated radius was 0.23 ± 0.01 nm where the positronium state was present. This was confirmed using the magnetic quenching phenomenon. The values of the positron lifetime found and the measured Doppler broadening of the annihilation line indicated unexpected behaviour of positrons in this mineral.