Primary silicon crystals in hypereutectic aluminum-silicon alloy castings frozen at moderate rates have been studied metallographically. Etched growth traces parallel to external facets were used to demonstrate that the majority of the crystals, though of widely varying external shape, grow in basically octahedral form, enclosed in {111} facets. Equations are developed that predict the limits within which true octahedral or spinel-type growth can occur. The equations are tested by measurements of growth trace spacings. Outside the predicted limits, there can be accelerated corner growth, which may result in hollow hopper crystals or even a dendritic form. Alternatively, a corner may be truncated by formation of a facet other than {111}.

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