The "classic data" leaves a lot to be desired. There are numerous well-documented examples of silica concentrations much higher than the limits shown on the classic graph. The most famous are, perhaps, certain volcanic waters and oil field produced waters where reactive silica is fully soluble and stable at ambient temperatures and at neutral pH at concentrations exceeding 300 ppm! At L*A, we encountered a wastewater sample from a demineralizer in Hawaii where the silica concentration in the neutralized wastewater was close to 1000 ppm. Although the silica polymerized
over time, it remained happily in solution.