baryta (Barium Sulfate) is the common whitening agent for papers (for paints Titanium Dioxide is used)
TiO2 is very common in paper too. There are blends of TiO2 with other whitening agents to get a better price/performance. Kaolin is used as a cheap whitening agent and creates a smoother surface. The cat litter like stuff to trap the inkjet inks are minerals. Called micro ceramics and other fancy names.
Canson probably used more expensive whiteners for the Rag Photographique and asks a price for it. Not all Canson papers are at that level and not all have their origin at Canson, so some papers are overpriced there. Moab Entrada Natural shows an almost identical spectral plot to the Rag Photographique, use of good whitening agents and no FBAs. At a nice price.
met vriendelijke groeten, Ernst
New: Spectral plots of +250 inkjet papers:http://www.pigment-print.com/spectralplots/spectrumviz_1.htm