  The quest for hard and tough transparent oxide glasses is at the core of
glass science and technology. Aluminosilicate glasses exhibiting nanoscale
phase separation emerge as promising candidates for such materials.
Nevertheless, proper control of the phase separation represents a daunting
challenge due to its elusive origins. Here we employ large-scale molecular
dynamics simulations and structural analysis to unravel the underlying
mechanisms of the phase separation in aluminosilicate. The observed phase
separation originates from an arrangement of SiO$_4$ and AlO$_n$ polyhedra,
which manifests from the second coordination shell and extends to higher
shells. This specific arrangement is driven by repulsion between the polyhedra,
reaching its maximum at around 50 mol% of Al$_2$O$_3$. This behavior becomes
pronounced around and below the glass transition temperature. This work sheds
light on the origin of phase separation and provides a route for further
exploration across other compositions to develop glasses with adapted
mechanical performance.
