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Allophane-like materials in the weathered zones of Silurian phosphate-rich veins from Santa Creu d'Olorda (Barcelona, Spain)

Published online by Cambridge University Press:  09 July 2018

M. C. Moro ,
M. L. Cembranos  and
A. Fernandez
M. C. Moro
Affiliation:
Departamento de Geología de la Universidad de Salamanca. 37008 Salamanca, Spain
M. L. Cembranos*
Affiliation:
Departamento de Geología de la Universidad de Salamanca. 37008 Salamanca, Spain
A. Fernandez
Affiliation:
Departamento de Geología de la Universidad de Salamanca. 37008 Salamanca, Spain
*
*E-mail: cembranos@gugu.usal.es
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Abstract

Allophane-like materials occur in the weathered zones of the phosphate-rich veins hosted in Silurian metasediments of the Catalonian Coastal Ranges. These metasediments also host sulphide and phosphate sedimentary mineralizations. Mineralogical and geochemical investigations of the allophanic samples indicate that they comprise Si-rich allophane, with a molar SiO2/Al2O3 ratio ranging between 1.19 and 2.23, with amorphous Al-(Ca) phosphate and hydroxylapatite as major minerals, and minor goethite and quartz. It is assumed that allophane, amorphous Al-(Ca) phosphate and hydroxylapatite come from the reaction of acid solutions, released during the weathering of sulphide interbedded in black shales, with phosphate-rich veins and volcano- sedimentary host rocks. Silica-alumina gels were deposited in fissures and cavities left by a previous dissolution of the phosphate-rich veins. Later the phosphate minerals filled the conchoidal microfractures and shrinkage microcracks of the allophane.

Keywords

allophanehydroxylapatitephosphate-rich veinsSpain
Type
Research Article
Information
Clay Minerals , Volume 35 , Issue 2 , April 2000 , pp. 411 - 421
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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