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Zeolitization of a Phonolitic Ash Flow by Groundwater in the Laach Volcanic Area, Eifel, Germany

Published online by Cambridge University Press:  01 January 2024

Franz Bernhard  and
Ulrike Barth-Wirsching
Franz Bernhard*
Affiliation:
Institut für Technische Geologie und Angewandte Mineralogie, Technische Universität Graz, Rechbauerstraße 12, A-8010 Graz, Austria
Ulrike Barth-Wirsching
Affiliation:
Institut für Technische Geologie und Angewandte Mineralogie, Technische Universität Graz, Rechbauerstraße 12, A-8010 Graz, Austria
*
*E-mail address of corresponding author: [email protected]
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Abstract

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Field and experimental studies were performed to understand the formation conditions of the Nettetal zeolite deposit, Laach volcanic area, Germany. This deposit shows pronounced small- (cm) and large-scale (tens of meters) variations of zeolitization, despite the same phonolitic precursor glass throughout the occurrence. Zeolitization of the pyroclastic ash flow is restricted to three distinct layers that are 0.15 to 10 m thick and separated by fresh ash. The glassy matrix is altered to chabazite, phillipsite, analcime and K-feldspar in various combinations, whereas the pumice clasts are altered predominantly to chabazite. Mass changes during zeolite formation appear to be small, and Ca enrichment in chabazite and phillipsite may have occurred after their formation by cation exchange.

The zeolites and zeolite assemblages observed in the Nettetal deposit were experimentally reproduced by reacting the phonolitic glass at 100–200°C with distilled water and 0.01 M alkaline solutions as well as with varying solid/liquid ratios and grain-sizes. Chabazite and phillipsite represented metastable transition phases with respect to analcime and K-feldspar. A high solid/liquid ratio accelerated the conversion of glass to zeolites.

None of the classic models of zeolite formation is fully applicable to the Nettetal deposit. The most probable environment for zeolitization in this deposit is the stagnant fringe water zone immediately above the groundwater table. In this zone, representing a relatively closed system, favorable solution compositions for zeolite formation could have been developed rather quickly by glass-water interaction, which is not possible within the more thoroughly flushed deeper parts of the groundwater system. The three distinct zeolite layers are probably the result of temporarily changing groundwater levels.

Keywords

Ash FlowEifelExperimental ZeolitizationFringe Water ZoneGroundwaterLaach Volcanic AreaPhonolitic GlassZeolites
Type
Research Article
Information
Clays and Clay Minerals , Volume 50 , Issue 6 , December 2002 , pp. 710 - 725
Copyright
Copyright © 2002, The Clay Minerals Society

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