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, 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, (K_0.80Na_0.05Ba_0.01)(Fe_11.29 ²⁺Mg_1.11Mn_0.25Fe_0.14 ^{3 +}Cr_0.01Al_0.19Ti_0.01)[Al_1.23Si_16.77O₄₂](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.