Electron probe microanalyses of zoisites from high-grade metamorphic rocks in Galicia, NW Spain have been obtained. The elements Na, Mg, P, K, Ti, and Mn were not detected above the 0.1 wt. % level. Assuming all Fe is present as Fe3+, a clear relation is obtained between the Fe content and the optic axial angle. Zoisite with 1.6 wt. % Fe2O3 proves to be uniaxial at 589 nm; uniaxiality in blue light (470 nm) lies at 1.5 wt. % Fe2O3. A change of the optic sign, from positive to negative, is obtained at 2.9 wt. % Fe2O3. The presence of minor elements greatly affects this relation, as is indicated by analyses of sector-zoned Sr-bearing zoisites of a muscovite-zoisite pegmatite from the same region. Under retrograde conditions Fe-rich α-zoisite reacts to form less Fe-rich α-zoisite or β-zoisite and clinozoisite.
