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Fully oxidized chromite in the Serra Alta (South Portugal) quartzites: chemical and structural characterization and geological implications

Published online by Cambridge University Press:  05 July 2018

Jorge Figueiras  and
Joāo C. Waerenborgh
Jorge Figueiras
Affiliation:
Dept. de Geologia, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, Ediffcio C2, 5° Piso, P-1700 Lisboa, Portugal
Joāo C. Waerenborgh
Affiliation:
ITN, Dept. de Química, P-2686 Sacavém Codex, Portugal
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Abstract

Several quartzite bodies outcrop along the Ferreira-Ficalho Thrust Fault (South Portugal), a major accident of the Iberian Variscan Orogen. The sediment is a very pure quartz sandstone, with trace amounts of ultra-resistant heavy minerals and chromite. Chemical characterization (microprobe analyses and Mössbauer spectroscopy) showed the chromite to be unique: besides being Zn-rich, complexly zoned and a cation deficient spinel, all the iron was found to be fully oxidized to Fe3+. Structure refinement of single-crystal X-ray diffraction intensities unambiguously identifies the mineral as a chromite and the Mössbauer data are consistent with tetrahedrally coordinated Fe3+ in the spinel structure. Current geodynamical models see the Ferreira-Ficalho Thrust Fault as a first-order suture resulting from a complex collision of two distinct continental blocks with partial obduction of the intervening oceanic crust. The chromite grains could be envisaged as remnants of an early erosion of this obducted oceanic crust, but its unique chemical character does not allow any definite conclusion. Yet, the complete quartzite heavy mineral contents and its petrographic features are not consistent with their deposition within a continental collision situation.

Keywords

chromitequartziteoxidationSerra AltaPortugal
Type
Research Article
Information
Mineralogical Magazine , Volume 61 , Issue 408 , October 1997 , pp. 627 - 638
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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