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Clays and Clay Minerals; February 2004; v. 52; no. 1; p. 106-114; DOI: 10.1346/CCMN.2004.0520110
© 2004 Clay Minerals Society
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FORMATION OF NONTRONITE FROM OXIDATIVE DISSOLUTION OF PYRITE DISSEMINATED IN PRECAMBRIAN FELSIC METAVOLCANICS OF THE SOUTHERN IBERIAN MASSIF (SPAIN)

J. C. Fernández-Caliani1,*, E. Crespo2, M. Rodas2, J. F. Barrenechea2 and F. J. Luque2

1 Departamento de Geología, Facultad de Ciencias Experimentales, Universidad de Huelva, 21071 Huelva, Spain
2 Departamento de Cristalografía y Mineralogía, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid, Spain

* E-mail address of corresponding author: caliani{at}uhu.es

This paper describes a rare occurrence of nontronite associated with sulfide-bearing felsic metavolcanics, providing evidence of colloidal deposition in open spaces as result of a low-temperature water-rock interaction. Microbotryoidal masses of green nontronite with impurities of kaolinite, illite, barite, amorphous silica and iron oxyhydroxides are found as vein and cavity fillings in deeply kaolinized rhyolites and rhyolitic tuffs of Precambrian age, at Oliva de Mérida in SW Spain. Clay mineral characterization has been carried out by X-ray diffraction, infrared spectroscopy, thermal analysis, analytical electron microscopy and stable isotope (oxygen and hydrogen) analysis. Nontronite was formed under low-temperature alteration conditions, from a continuous sequence of reactions and aqueous solution compositions, involving two basic processes that acted in concert: oxidative dissolution of pyrite and hydrolysis of K-feldspar. After acidity neutralization, dissolved silica released by incongruent dissolution of K-feldspar reacted with ferric sulfate derived from pyrite oxidation to form nontronite under oxidizing conditions, in the presence of relatively warm meteoric water.

Key Words: Felsic Metavolcanics • K-feldspar Hydrolysis • Nontronite • Pyrite Oxidation • Smectite • Spain • Weathering






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