Blueschists of the Shuksan unit can be related to greenschists from Otago by the following general reaction: crossite + epidote = albite + actinolite + iron oxide + H2O Greenschists within the Shuksan unit lack sufficient ferric iron to develop sodic amphibole. Ferric-iron–rich greenschists in Otago contain iron oxide rather than sodic amphibole.
Complete solid solution between sodic amphibole and actinolite is observed, and zoning in this composition range is common. Gradation from actinolite core to sodic amphibole rim is interpreted, from the observed phase relations, to represent growth during decreasing T/P; and rims of actinolite over sodic amphibole, growth during increasing T/P.
The composition of sodic amphibole is related to the mineral assemblage in which it occurs, as well as to the physical conditions of metamorphism. Invariant and univariant compositions of sodic amphibole are defined for assemblages containing quartz + albite + epidote + muscovite plus combinations of chlorite, stilpnomelane, magnetite, and hematite.