tridymite

[trid-uh-mahyt]
noun Mineralogy.
a polymorph of quartz occurring in the form of small crystals, commonly twinned, in siliceous volcanic rocks.

Origin:
1865–70; < German Tridymit, equivalent to tridym- (Greek trídym(os) triple, equivalent to tri- tri- + ()dymos didymous) + -it -ite1

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tridymite

silica mineral, the stable form of silica (silicon dioxide, SiO2) at temperatures between 870 and 1,470 C (1,598 and 2,678 F); at lower temperatures it transforms to high-quartz, at higher to cristobalite. It has three modifications: high-tridymite, middle-tridymite, and low-tridymite. Tridymite forms thin hexagonal plates that are generally twinned, often in groups of three; its name alludes to this habit. It commonly occurs in igneous rocks, more abundantly than cristobalite, as in the trachytes of Rhineland-Palatinate, Germany; northern Italy; and in the Massif Central, France. Tridymite, also found in meteorites, has the same chemical composition as coesite, cristobalite, stishovite, lechatelierite, and quartz but has a different crystal structure. For detailed physical properties, see silica mineral (Table 2)

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Only six forms appear on this diagram, and there is considerable question whether tridymite is truly stable.
Cristobalite and tridymite are two other forms of crystalline silica.
Other types of silica include cristobalite and tridymite.
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