The iron content and optic axial angle in zoisites from Galicia, NW Spain

Abstract

ABSTRACT. Electron probe microanalyses of zoisites from high-grade metamorph, ic 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 Fe 3+, a clear relation is obtained between the Fe content and the optic axial angle. Zoisite with 1.6 wt. ~o Fe203 proves to be uniaxial at 589 rim; uniaxiality in blue light (470 nm) lies at 1.5 wt. Fe203. A change of the optic sign, from positive to negative, is obtained at 2.9 wt. ~o Fe_~O3. 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 ct-zoisite reacts to form less Fe-rich ~t-zoisite or fl-zoisite and clinozoisite.KnYWORDS: zoisite, metamorphism, optic axial angle, iron, Galicia, Spain.T HE existence of two types of zoisite with different optical orientations was first recognized by Termier (t898, 1900). The two types are named a-zoisite and fl-zoisite after the vibration direction which lies parallel to the crystallographic b-axis, along which zoisite is usually prismatically developed, a-zoisite has ct, t, and 7 parallel to b, a, and c, respectively; OAP parallel to (100), varying 2V~ and strong rhombic dispersion r << v. fl-zoisite has ~, t, and 7 parallel to a, b, and c, respectively; OAP parallel to (010) and perpendicular to the cleavage (100), varying 2Vr and rhombic dispersion r > v.These data are usually given in literature on zoisite mineralogy and in determinative tables (Myer, 1966;Saggerson, 1975;Tr6ger, 1982). Deer et al. (1967) and Battey (1975) give a reverse dispersion scheme for both ~-and fl-zoisites. Vogel and Bahezre (1965) note the existence of a zoisite with a large 2V~ with rhombic dispersion r >> v.The connection between the two types of zoisite and their ferric iron content has been much debated (Phillips and Griffen, 1981;Saggerson, 1975; Termier, 1900;Winchell and Winchell, 1959). A relation, which nowadays is generally accepted has been given by Myer (1966) , with later confirmation by Enami and Banno (1977). Myer analysed fiveCopyright the Mineralogical Society non-manganiferous zoisites and measured the optic axial angle 2Vr in Nao light; his data are given in fig. 1. ct-zoisite is generally considered to be the hightemperature zoisite, occurring as a primary mineral in eclogitic rocks of various compositions, whereas during retrograde metamorphism fl-zoisite forms (Raith, 1976;Richter, 1973;Vogel, 1967).The present study was undertaken to refine the extrapolated curves of Myer and of Enami and Banno. Seventeen zoisite-bearing samples were collected from Cabo Ortegal, Galicia, NW Spain. a-zoisite occurs as a stable phase, together with omphacite, garnet and brownish pargasitic hornblende in ~t-zoisite eclogites, ct-zoisite hornblende eclogite and ~-zoisite hornblende pyrigarnites. The primary metamorphic character of the mineral may be deduced from the fact that it oc...