Mullite (Mul) formation after high-T muscovite (Ms) breakdown has been studied in phyllosilicate-rich bricks. At T ≥ 900 °C Ms dehydroxylation is followed by partial melting that triggers the nucleation and growth of Mul acicular crystals. An analytical electron microscopy study reveals that the Mul is a 3:2-type with a (Al1.686Ti0.031Fe0.159Mg0.134)(Al2.360Si1.649)O9.82 formula and an O atom vacancy of x = 0.18. This is consistent with X-ray diffraction results [i.e., unit-cell parameters: a = 7.553(7), b = 7.694(7), and c = 2.881(1) Å, V = 167.45 Å3]. The initial stage of the process resulting in Mul growth followed the balanced reaction Ms → 0.275Mul + 0.667Melt + 0.244K2O + 0.01Na2O + 0.125H2O, yielding an alkali-poor peraluminous melt. H2O with K (and Na), which are lost along the (001) planes of dehydroxylated Ms, play a significant role as melting agents. The c-axes of the Mul crystals are oriented parallel to ms or to the symmetrically equivalent <310>ms zone axis, while the (120)mul or (210)mul planes are subparallel to (001)ms (TEM results). These systematic orientations point to epitaxial Mul nucleation and growth on the remaining Ms substrate, which acts as a template for Mul heterogeneous nucleation. Randomly oriented Mul growth is also observed during the late stages of the process (i.e., melt cooling). The epitaxial nature of Mul growth after dehydroxylated Ms melting minimizes the energy requirement for nucleation. In addition, the water released after Ms breakdown and the multicomponent nature of the melt enable this high-T aluminum silicate to grow at T ~ 900 °C, almost 100 °C below the SiO2-Al2O3-K2O ternary system eutectic (after a melt with an end-member Ms composition).