Publication: Formation of nontronite from oxidative dissolution of pyrite disseminated in Precambrian felsic metavolcanics of the southern Iberian Massif (Spain)
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2004
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Clay Minerals Society
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Abstract
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 Me´rida 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.