Cr-rich Mg-sapphirines, which are more aluminous in composition than R²⁺:R³⁺:Si of 7:9:3 and are therefore considered to be peraluminous, are shown to have an inverse linear relationship between octahedrally co-ordinated Cr and Al. This balanced substitution, which has not previously been described, may be used to explain the major chemical variations exhibited in the described minerals. Since the peraluminous Cr-rich sapphirines are idioblastic it is suggested that they are stable members of the assemblages within which they occur. Peraluminous sapphirine is usually considered to be metastable but in this instance it is possible that chromium may assist in stabilizing its development.