Feldspar saturation state in natural waters

Andri Stefánsson*, Stefán Arnórsson

*Corresponding author for this work

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82 Citations (Scopus)


The saturation state of feldspar minerals in natural waters ranging from 0°C to over 300°C was studied. Waters above 200°C have closely approached equilibrium with microcline and low-albite. This is consistent with the occurrence of these minerals as hydrothermal minerals in active geothermal systems with temperature in excess of some 200°C. The Na+/K+ activity ratio of geothermal waters with temperature as low as 50°C closely approaches that predicted from thermodynamic data for the reaction low-albite + K+ = microcline + Na+, suggesting that Na and K ion activities in these geothermal waters are controlled by simultaneous equilibrium with these two feldspars. Geothermal waters are undersaturated with primary disordered plagioclases and alkali-feldspars of compositions typically found in volcanic rocks. Accordingly these feldspars tend to dissolve in such waters simultaneously with precipitation of ordered alkali-feldspars. Surface- and non-thermal groundwaters are usually undersaturated with respect to igneous and metamorphic feldspars regardless of composition and Al-Si ordering and tend, therefore, to dissolve under weathering conditions. Copyright (C) 2000 Elsevier Science Ltd.

Original languageEnglish
Pages (from-to)2567-2584
Number of pages18
JournalGeochimica et Cosmochimica Acta
Issue number15
Publication statusPublished - 2000

Bibliographical note

Funding Information:
The present study forms a part of a research project on water-rock interaction supported by the Research Fund of the University of Iceland and the Science Fund of the National Research Council, Iceland. It was initiated while the first author was at the University of Iceland but completed when he was at the ETH, Zurich, Switzerland. We are grateful to K. Akaku for the use of unpublished data on fluid composition. D. Larson is acknowledged for sharing with us unpublished data on feldspar composition in geothermal systems and fruitful discussions, B. Steingrı́msson for providing data on downhole temperatures. We are also indebted to I. Gunnarsson who carried out a large number of the speciation calculations. Further, S. R. Gı́slason, P. R. L. Browne, D. K. Nordstrom and G. Pokrovski are acknowledged for fruitful discussions during the course of this work. Reviews by three anonymous reviewers greatly improved the manuscript.


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