Geological and mineralogical research were conducted by the scanning probe microscopy and showed that in plagioclase pegmatites of North Baikal muscovite province the average thickness of the peristerites increases from early graphic zones to late trochlear zones and pegmatoid zones of pegmatite veins. The thickness reaches a maximum in the pegmatites of pegmatoid structure which contain large-crystalline muscovite. This regularity can be used for development mineralogical criteria for evaluation of pegmatite veins productivity at the large size muscovite source.