Iron in the minerals of the sodalite group

The study of the problem of the black coloration of feldspathoids of the sodalitic group (nosean, haüyne, sodalite) reveals iron as a main factor.A statistical study of the global compositions of these minerals proves that their enriching inFe is accompanied by a decrease in Al, but also by an increase in Si.A substitution of Fe⁺⁺⁺ to Al in the structure in the feldspathoids is suggested.The analysis with an electron microprobe confirms this hypothesis and allows us to state a fondamental difference between the formation of miaskitic phonolites rich in Fe⁺⁺ and that of agpaitic phonolites rich in Fe⁺⁺⁺.