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متالوژی::
کریستوبالیت
, other than the silica present in the form of aluminium silicate) changes into different polymorphs with rise in temperature such as beta- quartz at 574�C, tridymite at 870�C and crystobalite at 1400�C.
Although, the final form crystobalite is thermally stable, it is most often not reached below the firing temperature.
Consequently, clays containing high percentage of quartz tend to expand on heating and contract on cooling, and the ceramic products made out of such clay tend to develop cracks (only flint is transformed to crystobalite readily, and hence this variety of quartz is preferred for mixing with china clay in the feed).
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