Abstract

Zussmanite KFe 13 [AlSi 17 O 42 ](OH) 14 , a modulated 2:1 layer silicate, has so far been found only in ironrich metasediments from Laytonville. California (Agrell et al., 1965). A new occurrence is reported here from Punta Nihue north of Valdivia, Chile, in banded stilpnomelane-schists. These are intercalated in the "Western Series", a complex of low-grade metamorphic rocks with local high-pressure, low-temperature overprint (e.g. blueschists). The rock contains conspicuous porphyroblasts of zussmanite of mm size and is composed of chemically distinct bands with the subsequent assemblages: (1) zussmanite-stilpnomelane-quartz, (2) siderite-quartz+ or -stilpnomelane (3) apatite-stilpnomelane-quartz+ or -siderite. The chemical composition of zussmanite. (K (sub 0.80) Na (sub 0.05) Ba (sub 0.01) )(Fe (super 2+) (sub 11.29) Mg (sub 1.11) Mn (sub 0.25) Fe (super 3+) (sub 0.14) Cr (sub 0.01) Al (sub 0.19) Ti (sub 0.01) )[Al (sub 1.23) Si (sub 16.77) O (sub 4.2) ](OH) 14 , its optical properties and X-ray data correlate well with the Californian occurrence. Additionally, we present new IR data. In type (2) bands of fine-grained crystals of a K.Al poor mineral formed from siderite and quartz. Its chemical composition is close to that of zussmanite. A similar phase was also reported from Laytonville, California (Muir 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 stability field.

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