Green micas in the Archaean Isua and Malene supracrustal rocks, southern west Greenland, and the occurrence of a barian-chromian muscovite.

The green micas have a compositional range from nearly pure muscovite to types with up to approx 17 wt.% Cr₂O₃ and approx 8 wt.% BaO with variations in the concentrations of MgO and FeO^T (up to approx 3 wt.% each), TiO₂ (up to approx 2 wt.%) and Na₂O (up to approx 1 wt.%). Ba substitution on the mica interlayer or A site occurs continuously up to approx 0.22 cations/formula unit (11 oxygen normalization), whereas Cr substitution on the octahedral sites occurs continuously up to approx 0.45 cations/formula unit, followed by a compositional hiatus to types with approx 0.90 cations/formula unit. This hiatus may indicate a miscibility gap in the Cr-Al series. Ba substitution (Ba^AAl^iv = K^ASi^iv) is approximately balanced by Mg- and Fe-substitution (Mg,Fe)^viSi^iv = Al^viAl^iv) so that the tetrahedral site is = Si₃Al. The Ba and Cr may have originated from detrital chromite and sedimentary baryte in the sedimentary protolith, or be related to secondary processes.-W.S.W.