Nanometre-scale twins found in the triclinic crystalline phase devitrite, Na2Ca3Si6O16, were shown to exhibit the relatively rare form of twinning, Type II twinning, in which the crystal structure of one of the parts was related to the crystal structure of the other by a rotation of 180° about η 1 = [100]. Although formally, the twin plane K1 was irrational, it was in essence (010).

Type II Twinning in Devitrite, Na2Ca3Si6O16. K.M.Knowles, C.N.F.Ramsey: Philosophical Magazine Letters, 2012, 92[1], 38-48