Type II twinning in devitrite, Na₂Ca₃Si₆O₁₆

Abstract

Nanometre-scale twins found in the triclinic crystalline phase devitrite, Na₂Ca₃Si₆O₁₆, are shown to exhibit the relatively rare form of twinning, Type II twinning, in which the crystal structure of one of the parts is related to the crystal structure of the other by a rotation of 180° about η ₁ = [1 0 0]. Although formally, the twin plane K ₁ is irrational, it is in essence (0 1 0).

Keywords:

• crystallography
• devitrite
• transmission electron microscopy
• twinning

Acknowledgements

We are grateful to Professor M.A. Carpenter from the Department of Earth Sciences, University of Cambridge for undertaking resonant ultrasound spectroscopy between room temperature and 600°C on a sample of devitrified glass heat treated for one week at 920°C.