Definify.com

Webster 1913 Edition

Noun.

[Gr. [GREEK] threefold. So called in allusion to the supposed threefold nature of the twin crystals.]

(Min.)

Pure silica, like quartz, but crystallizing in hexagonal tables. It is found in trachyte and similar rocks.

Tridymite

tridymite ‎(plural tridymites)

(mineralogy) A rare mineral of volcanic rocks that solidified at a high temperature, with the chemical composition silicon dioxide, Si O₂. Tridymite is chemically identical to quartz, but has a different crystal structure. Sometimes contains sodium aluminum silicate, which seems to be expelled when quartz solidifies at lower temperatures.

mineral