Summary: Crystal-chemical calculations are based on three chemically analysed samples of ettringite and their respective unit-cell dimensions and specific gravities. The large hexagonal unit cell is deduced to contain 384 oxygen atoms, 64 cations with co-ordination number 6 or more, and 48 cations with co-ordination number 4 or less. Some substitution of cations with co-ordination number 3 apparently takes place, and any excess above 192 H₂O takes place in the form of substitution of (H₄O₄) for XO₄ anionic groups. The isomorphism is extremely complex but can be generalized in simple form as 16[A4(XO4)3˙(H2O)12] where A represents atoms with six-fold co-ordination (Ca, Na, Al) and X represents atoms with four-fold co-ordination (S, Si, H₄, and possibly some of the Al). It is implied that an isostructural series of silicon-containing calcium aluminium sulphate hydrates might exist, the extreme compositions of which are: 6CaO.Al₂O₃.3SO₃.3SiO₂.24H₂O and 6CaO.Al₂O₃.3SO₃.30H₂O.

The crystal structures of ettringite and thaumasite must be closely related.