Zussmanite in ferruginous metasediments from southern central Chile

Zussmanite KFe13[AlSi17O42](OH)(14), a modulated 2:1 layer silicate, has so far been found only in iron-rich metasediments from Laytonville, Californi a (Agrell ct al., 1965). A new occurrence is reported here from Punta Nihue north of Valdivia, Chile, in banded stilpnomelane-schists. These are inter calated in the 'Western Series', a complex of low-grade metamorphic rocks w ith local high-pressure, low-temperature overprint (e.g, blueschists). The rock contains conspicuous porphyroblasts of zussmanite of mm size and i s composed of chemically distinct bands with the subsequent assemblages: (1 ) zussmanite-stilpnomelane-quartz, (2) siderite-quartz +/- stilpnomelane (3 ) apatite-stilpnomelane-quartz +/- siderite. The chemical composition of zu ssmanite, (K0.80Na0.05B0.01)(Fe11.292+Mg1.11Mn0.25Fe0.143+Cr0.01Al0.19Ti0.0 1)[Al1.23Si16.77O42](OH)(14), its optical properties and X-ray data correla te well with the Californian occurrence. Additionally, we present new IR da ta. In type (2) bands of fine-grained crystals of a K,Al poor mineral forme d from siderite and quartz. Its chemical composition is close to that of zu ssmanite. A similar phase was also reported from Laytonville, California (M uir Wood, 1980). The rarity of rock-forming zussmanite can be explained by its occurrence in strongly Fe-rich and reduced rocks, as well as, by a possibly narrow P-T s tability field.