| 1. | Silification is prominent and has formed cristobalite and tridymite.
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| 2. | Thus although tridymite and cristobalite are common minerals in ignimbrites, they may not be primary magmatic minerals.
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| 3. | Minerals associated with this include quartz, cristobalite and tridymite, which may all be present in volcanic ash.
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| 4. | The high-temperature minerals, cristobalite and tridymite, have both lower densities and indices of refraction than quartz.
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| 5. | In the older trachytes, secondary quartz is not rare, and probably sometimes results from the recrystallization of tridymite.
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| 6. | Commonly in most felsic ignimbrites the quartz polymorphs cristobalite and tridymite are usually found within the welded tuffs and breccias.
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| 7. | Other phases, e . g . the higher-temperature phases tridymite and cristobalite, are not significant for oscillators.
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| 8. | Quartz is typically rare in trachyte, but tridymite ( which likewise consists of silica ) is by no means uncommon.
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| 9. | Polymorphism can also result from the existence of different ?-quartz, tridymite, cristobalite, coesite, and stishovite.
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| 10. | Quartz has several polymorphs, including tridymite and cristobalite at high temperatures, high-pressure coesite, and ultra-high pressure stishovite.
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