On suitably heat treated samples of float glass, devitrite, Na2Ca3Si6O16, nucleates heterogeneously on the glass surface and grows as thin needles with a distinct  growth axis. The needles group together as spherulites and show evidence of low-angle branching in order to fill space. Analysis of the rate of needle growth along  between 680°C and 950°C suggests that the activation energy for diffusion governing the transport across the devitrite–glass interface is 260 kJ mol–1, somewhat higher than previous analyses have suggested. The devitrite needles are often twinned, with the twins related to one another by a Type II twinning rotation of 180° about the  growth axis direction. The formally irrational twin plane is very close to (010). Molecular dynamics simulations have been undertaken to establish the form of these twin interfaces to understand how and why devitrite twins in this manner.