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Mineralogical Magazine; December 1998; v. 62; no. 6; p. 869-876
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Zussmanite in ferruginous metasediments from Southern Central Chile

H. J. Massonne, F. Herve, O. Medenbach, V. Munoz, and A. P. Willner

Universitaet Stuttgart, Institut fuer Mineralogie und Kristallchemie, Stuttgart, Federal Republic of Germany

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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A. P. Willner, S. Pawlig, H.-J. Massonne, and F. Herve
METAMORPHIC EVOLUTION OF SPESSARTINE QUARTZITES (COTICULES) IN THE HIGH-PRESSURE, LOW-TEMPERATURE COMPLEX AT BAHIA MANSA, COASTAL CORDILLERA OF SOUTH-CENTRAL CHILE
Can Mineral, December 1, 2001; 39(6): 1547 - 1569.
[Abstract] [Full Text] [PDF]


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