Nano- to micro-scale lamellae of ferro-aluminian phlogopite (Phl) within centimetre-size crystals of muscovite (Ms-matrix) from a pegmatite near Gorduno (Lepontine Domain, Central Alps, Switzerland) were discovered using high-resolution transmission (HRTEM) and analytical electron microscopy (AEM). The Msmatrix is a 2M1 polytype with occasional disordered sequences. The Phl lamellae, covering a large range of sizes, are parallel to the basal plane of the host. The interfaces parallel to (001) are straight, whereas the interfaces perpendicular to the basal plane are irregular. The lamellae are concentrated on levels of the Ms-matrix with disordered stacking. The Ms-matrix has close to end-member composition with 0.15 tri-octahedral cations p.f.u. The Phl-lamellae show considerable di-octahedral and eastonite substitution and are zoned, with rims slightly richer in di-octahedral component. The Ms-matrix itself is homogeneously exsolved into two sets of nanometre-sized lamellae at an angle of about 40° to the basal plane.
The Phl-lamellae and the fine, homogeneously nucleated lamellae in the Ms-matrix are exsolutions of a primary pegmatitic muscovite, richer in tri-octahedral component. Micas along the di-tri-octahedral join exsolve with increasing temperature and the composition of the exsolved matrix points to an equilibration temperature of 600°C. The recalculated primary composition points to a crystallisation temperature around 500°C. The nearby, younger Novate intrusion body probably acted as heat source for the post-magmatic increase of temperature.