The groundmass of these rocks is often aphanitic microcrystalline, with a web of minute feldspars mixed with interstitial grains of quartz or tridymite; but in many dacites it is largely vitreous, while in others it is felsitic or cryptocrystalline.
32.
In July 2015, the same rover identified tridymite in a rock sample from Gale Crater, leading scientists to believe that silicic volcanism might have played a much more prevalent role in the planet's volcanic history than previously thought.
33.
A common feature of glassy rocks is the presence of rounded bodies ( spherulites ), consisting of fine divergent fibres radiating from a center; they consist of imperfect crystals of feldspar, mixed with quartz or tridymite; similar bodies are often produced artificially in glasses that are allowed to cool slowly.
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Veining can be reduced or avoided by the use of more refractory non-silica sands ( zircon, chromite etc ); by flux additives which lower the temperature of silica transition to tridymite or cristobalite, or which sinter the sand increasing resistance to failure; or by organic additives which are decomposed to carbon at high temperature, which then bonds to silica increasing strength, imparting veining resistance.
35.
"' Seifertite "'is a Martian where it is presumably formed from either tridymite or cristobalite other polymorphs of quartz as a result of heating during the atmospheric re-entry and impact to the Earth, at an estimated minimum pressure of 35 GPa . It can also be produced in the laboratory by compressing cristobalite in a diamond anvil cell to pressures above 40 GPa . The mineral is named after Friedrich Seifert ( born 1941 ), the founder of the Bayerisches Geoinstitut at University of Bayreuth, Germany, and is officially recognized by the International Mineralogical Association.
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