Computer-controlled scanning electron microscopy (CCSEM) and X-ray diffraction (XRD) were used to investigate Si-rich phases in several coal fly ash (CFA) samples. The CCSEM measurements utilized both particle size distributions and a particle shape parameter, circularity, to classify the Si-rich phases in these ashes. The results indicated that the amount of free, respirable, crystalline quartz in these CFA samples was very small (< 1.0 vol.%). Neither the total quartz nor the respirable quartz determined by CCSEM showed a significant correlation with the XRD results for the bulk ash. (C) 2007 Elsevier B.V. All rights reserved.