Summary: The mole fractions of the silicons present in melilites as monomeric (SiO₄) and dimeric (Si₂O₇) groups depends both upon the composition of the melilite and also upon the degree of Al-Si ordering. It is shown that the absolute numbers of monomeric and dimeric silicons in a 10 mg polycrystalline sample can be determined by a relatively rapid chromatographic method.

A statistical theory is developed to treat the data; this contains a variable κ-parameter that describes the degree of ordering.

Experimental results are presented and interpreted for one natural and five synthetic melilites. The synthetic gehlenite-rich melilites are appreciably, but not completely disordered. The natural melilite is well ordered, but heating at 1050 °C introduces considerable disorder.