Brightness (R457) is the main quality parameter in the production of ground calcium carbonate (GCC) slurries for the paper industry. In order to study how the presence of trace amounts of pyrrhotite (Fe(1-x)S) and pyrite (FeS2) affect the brightness of GCC slurries, experiments were performed in which a high quality calcite concentrate was spiked with these sulphides prior to micronisation and subsequent spectrophotometric measurements. The results show that even very small additions (<0.05 wt%) have a significant detrimental effect on the brightness of the product. Pyrrhotite reduces the brightness of the product more than pyrite, probably as an effect of differences in inherent optical reflectivity, specific surface area after micronisation and smearing. In addition, the experiment shows that when brightness is presented as a function of iron concentration, magnetite (Fe3O4) produces a different result compared to the sulphides. This indicates that a simple iron analysis is not sufficient to predict brightness of the GCC slurry when several iron-bearing minerals are present. The particle size distribution of the contaminant phase has proven to have a significant effect, as brightness is reduced when the contamination becomes more finely distributed. (C) 2013 Elsevier Ltd. All rights reserved.