Nov 01, 1995 · Sandstone (Type I), by definition, consists of mineral and rock fragments containing at least 60% free silica, within the sand size range of to 2 mm. Quartzitic sandstone (Type II), which is harder, contains at least 90% free silica; and quartzite Type III), a metamorphic rock, is the hardest quartz based stone, with at least 95% free silica.
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