The Iron Content and the Optic Axial Angle in Zoisites from Galicia, NW Spain

P. Maaskant
Instituut voor Aardwetenschappen, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

Abstract: 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.

Keywords: zoisite • metamorphism • optic axial angle • iron • Galicia • Spain

Mineralogical Magazine; March 1985 v. 49; no. 350; p. 97-100; DOI: 10.1180/minmag.1985.049.350.14
© 1985, The Mineralogical Society
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