Physics
Scientific paper
Aug 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.3345g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 16, pp.3345-3351
Physics
Scientific paper
The reconstruction of redox conditions in ancient environments is difficult in the absence of authigenic Fe and Mn phases as indicators. The Miocène sepiolite beds in the Madrid Basin (central Spain) that were formed in closed Systems under arid to semiarid climate are examples of an environment poor in Fe and Mn phases. In this paper, a new approach is presented that uses structure-bound transition metal ions in sepiolites from two localities (Vallecas and Valdemoro) to deduce redox conditions during their formation. Scanning electron microscopy revealed that most of the sepiolite is fibrous and that a subordinate part has a spheroidal morphology. Electron spin resonance spectroscopy showed Fe(III), V(IV), and V(III) located in octahedral sites of the sepiolite structure. Bound V(III) indicates sulfidic conditions during the crystallization of the sepiolite fibers that form the major part of the deposits. The cessation of the formation of sepiolite deposits is indicated by the poorly crystallized spheroidal morphotype. The occurrence of Mn-oxides in the pore space of the sepiolite deposits indicates later oxic conditions that led to a partial oxidation of the metal centers in the sepiolite structure.
Fry I. V.
Gehring Andreas U.
Luster J.
Sposito Garrison
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