Une enquête sur les dépôts de silice de Southern Illinois . et etc. peu cristobalite ar:d tridymite sont trouvés. Par properheat, traitement/hov jamais, quartz, tridymite, cristobalite ou peuvent être produits.*butnot la variété cryptocristalline. La température à laquelle ces inversionstake place sont :-, présence de tjhe. un flux ^ Inversiona - Quartz b Qua.rtzb Tridymiteb - Quartz bg2 Tridymite - b Cristobalitea Tridymitehi Tridymite - bi Tridymite - b2 5750 C 8700 Température Tridymite-lOOC1470°-10°C117^0 Remarques.et rapide.réversible réversible et lent.Lent et rapide et réversible.Reversibl

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An investigation of the silica deposits of southern Illinois . and etc. Little cristobalite ar:d tridymite are found. By properheat treatment, hov/ever, quartz, tridymite, or cristobalite can be produced.*butnot the cryptocrystalline variety. The temperatures at which these inversionstake place are:- In, tjhe presence of. a Flux ^ Inversiona Quartz - b Qua.rtzb Quartz - bg Tridymiteb2 Tridymite - b Cristobalitea Tridymite - bi Tridymitehi Tridymite - b2 Tridymite Temperature 5750 C 8700-lOOC1470°-10°C117^0 Remarks.Rapid and Reversible.Slow and Reversible.Slow and Reversible.Rapid and Reversible.Rapid and Reversible. 1630C When no flux is present the bQuartz to bgTridymite inversion does notoccur and as the temperature reaches 1250°C, bcristobalite fonis. With tridy-mite and no flux, cristobalite does not form until 1570°C is reached. Chert andchalcedony change over to tridymite at a much greater speed than quartz due to^D.Ross - B.of.S. Tech. Paper 116 - p. 67 ^C.N.Fenner - Am.Jr.Sc. Vol 36 - Page 339 - Stability Relations of SilicaMaterials.. 4. its finer grained structure. These various forms of silica have different specific gravities, whichmeans that these changes in form are accompanied by volume changes. The specificgravities range as follows:- Thus a complete inversion of quartz to tridymite would cause a true volumeincrease of 16*S%» This, hov/ever, is theoretical for only after prolonged heatcould this state be reached. The occurrence of these inversions thereforecauses trouble in the manufacture of silica brick, and often causes apparentlyexcellent materials to be worthless for refractories. The sluggishness of someof the inversions, which permit the production of brick which at ordinary temp-eratures are made up of the forms of silica, stable at high temperatures, isthe factor that makes silica a valuable refractory. The fact that silica minerals change their form at various temperaturesand that this change proceeds mare rapidly and v/ith different e

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