For example, quartz will change into a variety of its SiO 2 polymorphs, such as tridymite and cristobalite at high temperatures, and coesite at high pressures.
22.
The temperature at which the liquidous phases crystallize in a sequence : mullite + tridymite, followed by sekaninaite and finally fayalite + clinoferrosilite ( Grapes, 2010 ).
23.
The bedrock of the district is made up of at least five rock types, the most prevalent of which are composed of porphyritic, tridymite-bearing rhyolite.
24.
Silicosis is due to deposition of fine respirable dust ( less than 10 micrometers in diameter ) containing crystalline silicon dioxide in the form of alpha-quartz, cristobalite, or tridymite.
25.
It occurs associated with hematite, magnetite, bixbyite, ilmenite, enstatite-ferrosilite, tridymite, quartz, sanidine, topaz, spessartine, beryl, mica, cassiterite and apatite.
26.
Silica ( the chemical compound SiO 2 ) has a number of distinct crystalline forms : quartz, tridymite, cistobalite, and others ( including the high pressure polymorphs Stishovite and Coesite ).
27.
The persistence of cristobalite outside of its thermodynamic stability range occurs because the transition from cristobalite to quartz or tridymite is " reconstructive ", requiring the breaking up and reforming of the silica framework.
28.
Silicon oxygen bond lengths vary between the different crystal forms, for example in ?-quartz the bond length is 161 pm, whereas in ?-tridymite it is in the range 154 171 pm.
29.
Fe-rich olivine is relatively much less common, but it occurs in igneous rocks in small amounts in rare granites and rhyolites, and extremely Fe-rich olivine can exist stably with quartz and tridymite.
30.
The groundmass contains interstitial quartz or tridymite or cristobalite . " Olivine tholeiite " has augite and orthopyroxene or pigeonite with abundant olivine, but olivine may have rims of pyroxene and is unlikely to be present in the groundmass.
