Granular materials size segregate when exposed to external periodicperturbations such as vibrations. Moreover, mixtures of grains of differentsizes spontaneously segregate in the absence of external perturbations: when amixture is simply poured onto a pile, the large grains are more likely to befound near the base, while the small grains are more likely to be near the top.Here, we report a spontaneous phenomenon arising when we pour a mixture betweentwo vertical plates: the mixture spontaneously stratifies into alternatinglayers of small and large grains whenever the large grains are rougher than thesmall grains. In contrast, we find only spontaneous segregation when the largegrains are more rounded than the small grains. The stratification is related tothe occurrence of avalanches; during each avalanche the grains comprising theavalanche spontaneously stratify into a pair of layers through a "kink"mechanism, with the small grains forming a sublayer underneath the layer oflarge grains.
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