Tridymite definition

The name refers to the peculiar habit of crystallizing in groups of three individuals.

tridymite (plural: tridymites)

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.