Zussmanite in Ferruginous Metasediments from Southern Central Chile

H.-J. Massonne1, F. Hervé2, O. Medenbach3, V. Muñoz2 and A. P. Willner3
1 Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstr. 18, D-70174 Stuttgart, Germany
2 Departamento de Geologia, Universidad de Chile, Casilla 13518, Correo 21, Santiago de Chile, Chile
3 Institut für Mineralogie, Ruhr-Universität, D-44780 Bochum, Germany

Abstract: Zussmanite KFe13[AlSi17O42](OH)14, a modulated 2:1 layer silicate, has so far been found only in iron-rich 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±stilpnomelane (3) apatite-stilpnomelane-quartz±siderite. The chemical composition of zussmanite, (K0.80Na0.05Ba0.01)(Fe11.292+Mg1.11Mn0.25Fe0.143+Cr0.01Al0.19Ti0.01)[Al1.23Si16.77O42](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.

Keywords: zussmanite • metasediments • schist • Punta Nihue • Chile

Mineralogical Magazine; December 1998 v. 62; no. 6; p. 869-876; DOI: 10.1180/002646198548098
© 1998, The Mineralogical Society
Mineralogical Society (www.minersoc.org)