Ferruginous concretions in a Late Cretaceous karst bauxite; composition and conditions of formation

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doi: 10.1016/S0009-2541(99)00061-3
Authors:Mongelli, G.; Acquafredda, P.
Author Affiliations:Primary:
Universita della Basilicata, Centro di Geodinamica, Potenza, Italy
Other:
Universita di Bari, Italy
Volume Title:Chemical Geology
Source:Chemical Geology, 158(3-4), p.315-320. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1999
Note:In English. 15 refs.; illus.
Summary:The karst bauxites from the Apulian carbonate platform (southern Italy) have an ooidic texture. The concretions, which are formed at the expense of the kaolinitic matrix, are composed of Al-hematite in the core and of Al-hematite and boehmite in the banded cortex. The Al-hematite in the core has an average formula (Al0.12Fe0.88)2O3 and the Al is within the range of values observed in hematite formed through in situ epigenetic replacement of kaolinite. The Al substitution in hematite involves a water-unsaturated environment, possibly promoted by fluctuations of the groundwater table followed by intense drainage and evaporation. In the banded cortex, boehmite may form instead of Al-hematite at lower water activity values. Further, the boehmite stability increases for high values of the aAl3+/a3H+ ratio, which is mostly controlled by the pH of the solution. The stability of the Al-hydroxide decreases with increasing pH. Abstract Copyright (1999) Elsevier, B.V.
Sections:Geochemistry
Subsections:Sedimentary rocks
Subjects:Aluminum; Bauxite; Bauxitization; Boehmite; Concretions; Cretaceous; Crystal chemistry; Crystal growth; Ferruginous composition; Geochemistry; Hematite; Karst; Mesozoic; Metals; Oxides; Secondary structures; Sedimentary rocks; Sedimentary structures; Substitution; Textures; Upper Cretaceous; Apulia Italy; Europe; Italy; Southern Europe
Abstract Numbers:00M/668
Record ID:1999045812
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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