Chloritoid and staurolite coexist with Al silicate, chlorite, or garnet + or - biotite in Precambrian quartzite and schist from northern New Mexico. The observed Fe-Mg reversal is not related to variable P, T, or minor element content, including Fe (super 3+) . However, it could arise from any of three factors: (1) Fe and Mg may occur on several crystallographic sites in one or both minerals; (2) some Mg may not be exchangeable with Fe in staurolite; or (3) Fe and Mg may mix non-ideally in one or both phases.--Modified journal abstract.

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