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Clays and Clay Minerals

, Volume 34, Issue 3, pp 338–345 | Cite as

Hydroxy-Cu-Vermiculite Formed By the Weathering of Fe-Biotites at Salobo, Carajas, Brazil

  • Philippe Ildefonse
  • Alain Manceau
  • Dominique Prost
  • Maria Christina Toledo Groke
Article

Abstract

Weathering of a copper stratiform deposit (schist) at Salobo, Brazil, has produced two distinct Cu-bearing minerals from a biotite parent: vermiculite and a manganese oxide containing as much as 13% and 25% CuO, respectively. Manganiferous products were formed as the result of an interhorizon transfer of solutions through a fissure system. Thus, the structural orientation of the schists was a major factor in controlling the supergene concentration of Cu. The Cu-vermiculite formed by the weathering of Fe-biotite, although the unweathered biotites in the parent rocks were found to contain no copper, suggesting that Cu was supplied by weathering solutions. X-ray powder diffraction (XRD) and cationexchange capacity data for the Cu-vermiculite differ from those of typical Mg-vermiculite and are similar to those of hydroxy- Al-vermiculite. A comparison of the XRD pattern of the Cu-vermiculite with that of a Cu-free vermiculite indicates that Cu atoms are located in interlayer sites. Cu probably occurs in a brucite-like layer. The position and structure of the Cu K-absorption spectrum suggest that the Cu is divalent and exists in 6-fold coordination.

Key Words

Cation-exchange capacity Copper Intergrade Manganese oxide Supergene enrichment Vermiculite Weathering 

Résumé

L’altération supergéne des schistes du dépôt stratiforme de cuivre de Salobo (Brésil) a produit deux phases minérales porteuses de cuivre, associées aux biotites: une vermiculite et des oxydes de manganèse contenant respectivement de 13% à 25% de CuO. Les produits manganésifères résultent de transferts interhorizons d’ions en solution dans le système fissural. Aussi, l’orientation structurale des schistes est un facteur majeur qui contrôle l’accumulation supergène du cuivre. La vermiculite Cu se forme à partir de l’altération des biotites Fe de la roche saine. Les biotites saines, cependant, ne contiennent pas de cuivre ce qui suggère que cet élément est fourni par les solutions d’altération. Les données de la diffraction des rayons X et les mesures de la capacité d’échange de cations obtenues à partir de la vermiculite Cu diffèrent notablement de celles de vermiculite Mg classique. Par contre, elles sont proches de celles des vermiculites hydroxy-alumineuses, bien connues par ailleurs. La comparaison des spectres de diffraction de la vermiculite Cu avec ceux de la vermiculite non cuprifère de la base du profil montre que les atomes de Cu sont localisés en sites interfoliaires. La position et la structure des spectres d’absorption X au seuil K du cuivre suggèrent que les atomes de cuivre sont divalents et en position hexacoordonnée.

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Copyright information

© The Clay Minerals Society 1986

Authors and Affiliations

  • Philippe Ildefonse
    • 1
  • Alain Manceau
    • 2
    • 4
  • Dominique Prost
    • 1
  • Maria Christina Toledo Groke
    • 3
  1. 1.Laboratoire de PédologieUniversité Paris 7Paris Cedex 05France
  2. 2.Laboratoire de Minéralogie-Cristallographie, UA 0-9Universités Paris 6 et 7Paris Cedex 05France
  3. 3.Institute di GeoscienciasUniversidad de Sao PauloSao PauloBrazil
  4. 4.Laboratoire pour l’Utilisation du Rayonnement Electromagnétique (LURE)C.N.R.S.OrsayFrance

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