The silica contents given in my earlier comment are taken from the Geological Survey of Great Britain’s ‘Geology of the country around Sheffield’ published 1957. On page 169 there are 2 ganister example chemical analysis. One shows SiO2 content of 97.02% and is described as Ganister, first-quality, from Nos 1 and 3 mines Beeley Wood (Analyst E.G. Radley). The other shows SiO2 content of 98.40% and is described as from Park Wood Mine, Neepsend (from Pickford Holland & Co).