An orthrohombic superstructure of tridymite existing between about 105 and 180°C

Abstract

A modification of tridymite with an orthorhombic structure exists between monoclinic low tridymite and orthorhombic high tridymite. The three unit-cell edges of this structure are parallel with the corresponding ones of orthorhombic high tridymite but the a translation is three times as long as that of the latter. The crystal structure was determined by the use of x-ray intensities at about 155°C on a crystal from refractory silica brick. The space group is P2₁2₁2₁ and the unit cell of, dimensions a = 26.171(3), b = 4.986(4) and c = 8.196(4) Å, contains twenty-four formula units. The six-membered rings of the silica tetrahedra are distorted in a way similar to those of monoclinic low tridymite: two thirds have a ditrigonal shape and one third have an oval shape in (001). The same ring types stack on top of one another along the orthorhombic c axis (parallel with the c direction of hexagonal ideal tridymite). This is not the case in monoclinic low tridymite. The mean Si–O distance, uncorrected for thermal motion, is 1.58 Å and the mean values of the Si–O–Si angles are 170.4° for tetrahedra joined nearly parallel to c, and 153.0° for those nearly in (001).