It was recently shown that silicon particles in heat-treated Al-Si casting alloys can contain flaws such as surface pinholes and grooves, which cause varying degrees of reduction in the in situ particle fracture strength and hence influence the mechanical properties of this class of alloys. In this work, we show that the formation of one class of such strength-limiting flaws in solidified and coarsened Si particles, namely surface pinholes, is caused by alloy impurities such as Fe and Ti in both binary eutectic Al-Si alloys and also in casting alloy A356. This is evidenced by using Focused Ion Beam serial sectioning tomography coupled with Energy-Dispersive X-Ray Spectroscopy, and confirmed by the observation that a high-purity Al-Si alloy presents a significantly lower proportion of pinholes along the surface of the silicon phase than does an alloy of commercial purity. A similar correlation between alloy purity and the formation of another, more severe strength-limiting particle defect, namely grooved interfaces, was on the other hand not found.