Chemically zoned phengite flakes from an impure marble of the ultra-high pressure Brossasco-Isasca Unit (Dora-Maira massif, Western Alps - Italy) were investigated by scanning, transmission and analytical electron microscopies (SEM-TEM-AEM), revealing a complex nano-structure within the crystal cores. Diffraction patterns show that this phengite is a highly ordered 3T polytype. A close inspection of the diffracted spots reveals that, for some classes of crystallographic indexes, weak satellite spots surround a central stronger spot. In bright-field TEM images, 60–120 Å wide areas of mottled contrast cross, at an angle of about 40°, the host phengite [Phe (a)] that represents the matrix. The satellite spots, the spatially ordered mottled contrast and the AEM analyses are all indicative of the presence of high-Si phengite relics [Phe (b)] (Si = 3.61 apfu) within the ordered matrix (Si = 3.38 apfu). The higher-Si phengite has the c cell parameter [29.68 Å; Phe (b)] different from that of the lower-Si matrix [29.75 Å; Phe (a)]. A zoned 3T phengite mantle (3.41-3.31 Si apfu) surrounds the phengite cores showing decreasing c values from the inner towards the outer part (i.e., 29.78-29.85 Å). A later 2M1 polytype with Si = 3.22 apfu has been locally detected at the edge of the phengite crystals. Both the crystallographic and crystal-chemical data of the higher-Si phengite relics, as well as petrologic information have been used to reconstruct prograde, peak and decompressional growth stages of the phengite flakes. Differences in both the octahedral and tetrahedral contents between the two types of phengite nano-domains suggest that the formation of the high-Si Phe (b) is due to an incomplete ionic reaction affecting the prograde Phe (a) at the metamorphic peak. The proposed ionic reaction is: 1 Phe (a) + 0.20 Mg2+ + 0.23 Si4+ → 1 Phe (b) + 0.44 Al3+.