Monoclinic tridymite was found in volcanic rocks from three localities; Yugawara, Harius and Mt. Myoho in Japan. The unit cell parameters of the Yugawara tridymite are a=25.89, b=5.00, c=18.59Å and β=117.7°, and space group is Aa or A2⁄a. This monoclinic tridymite was transformed to the form with n=10 by rapid cooling from high temperature.
