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a mineral that is one of the crystalline polymorphs of SiO2. Such trace elements as Fe, Al, and Na are commonly found in tridymite. It crystallizes in the form of colorless or gray tabular crystals, spherical aggregates, spear-shaped twins, and, often, trillings. It is a rare mineral and is usually encountered in cavities of juvenile extrusive acidic rocks and in tuffs; it is also found in meteorites and moon rocks. As a constituent of Dinas brick, tridymite determines the refractory properties of the brick.