Word Study
tridymite
CIDE DICTIONARY
tridymite, n. [Gr. threefold. So called in allusion to the supposed threefold nature of the twin crystals.].
Pure silica, like quartz, but crystallizing in hexagonal tables. It is found in trachyte and similar rocks. [1913 Webster]
OXFORD DICTIONARY
tridymite, n. a crystallized form of silica, occurring in cavities of volcanic rocks.
Etymology
G Tridymit f. Gk tridumos threefold (as TRI-, didumos twin), from its occurrence in groups of three crystals
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