Summary: Vlasovite, Na₂ZrSi₄O₁₁, previously reported as monoclinic by Pyatenko and Voronkov from an occurrence in the Kola Peninsula, Russia, has been found in rocks from Ascension Island in a triclinic twinned modification. The obliquity is very slight (α and β, differ from 90° by less than 1°), and X-ray photographs show arrays of paired spots corresponding to diffraction by different parts of the crystal related by the twinning operation, reflexion across (010).

On heating, this vlasovite is found to transform to the monoclinic variety at 29° C. The phase change has been followed on a single-crystal X-ray heating camera, and the obliquity is found to have successively smaller values as the crystal is heated from temperatures below room temperature to 29° C. The obliquity is characteristic of the particular temperature over the transformation range and is not affected by thermal history.

The transformation appears to be a displacive one, similar in some respects to α → β tridymite and α → β cristobalite, and some information has been obtained by X-ray structural investigations about the nature of the atomic movements involved.