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