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Mechanisms of Mineral Replacement Reactions

Final Report Summary - DELTA-MIN (Mechanisms of Mineral Replacement Reactions)

The report below is also attached as a pdf file.

Mechanisms of Mineral Replacement Reactions
Contact Details: Delta-Min Office, Prof. Andrew Putnis, Institut für
Mineralogie, Corrensstrasse 24, 48149, Münster, Germany
Project Website: www.delta-min.com
The scientific objective of the DELTA-MIN project was to significantly advance the understanding of fluid-induced mineral transformation mechanisms by exploring a wide range of diverse phenomena from a unified point of view. The project combined observations of fluid-induced replacement in natural rocks, experimental studies of mineral-fluid interaction and fundamental theory to understand dissolution, precipitation and mass transport. Observations ranged from the mega- to the nano-scale: from estimating the extent of metasomatism in rocks, to high resolution transmission electron microscopy of reaction interfaces.
The methods used to achieve these objectives included:
• In situ Atomic Force Microscopy to study crystal growth and dissolution mechanisms
• Hydrothermal experiments in various types of reactors to study mineral replacement mechanisms
• Dissolution rate studies in both flow-through and batch reactors
• Field work in areas where fluid-rock interaction can be studied on an outcrop scale
• Scanning and transmission electron microscopy to study reaction microstructures in both experimental and natural samples
Significant breakthroughs were made on a number of research problems:
• Atomic Force Microscopy of in situ crystal growth and dissolution mechanisms has demonstrated that background electrolytes can have a significant effect on kinetics by modifying water dynamics.
• Hydrothermal experimental studies coupled with observations from natural rocks have identified replacement mechanisms and mass transport pathways.
• The mechanism of leaching was unambiguously identified as a sequential process of congruent dissolution coupled with reprecipitation, in contrast to the accepted view of solid state interdiffusion.
• A unified concept of fluid-induced mechanisms of metasomatism and metamorphism in natural rocks has been published.
• Carbonation mechanisms of silicate rocks have been described from both experiments and studies of natural carbonated rocks.
• Dissolution rates of minerals have been related to surface crystallographic orientation, providing a a more mechanism-based interpreatation of rate data.
• The crucial importance of an interfacial boundary layer at the mineralfluid interface has been demonstrated experimentally, showing that
supersaturation in the bulk fluid is not a criterion for precipitation.
The overall conclusions of these results are that in multicomponent minerals fluid-mineral interactions take place by
(i) a dissolution step which usually results in a supersaturation in a boundary layer at the fluid-mineral interface
(ii) precipitation of a product phase which may take place within this boundary layer, and
(iii) establishment of a feedback between dissolution and precipitation.
This coupled process, which may be manipulated by changing the fluid composition, is the primary mechanism of mineral reequilibration in the presence of a fluid phase.
The applications of these results are a significant improvement in
• understanding how one rock type can be transformed to another
• using replacement mechanisms to produce more stable coatings on marble cultural heritage
• understanding how the reactions between phosphate fertilizer and lime-rich soils can limit the bioavailability of phosphate
• applying a better understanding of dissolution mechanisms to contribute to modelling long term stability in nuclear waste repositories
• applying "geo-inspired" fabrication of new materials by pseudomorphic replication.
The dissemination of the project was through peer-reviewed publications (40 publications in international ISI listed journals with more either already submitted or in preparation), through many international conference abstracts (~90) and presentations and convened special sessions in international conferences.
A highlight of the project was the Final International Conference held at the University of Oviedo, Spain from 28 February - 2 March 2012. This was attended by over 100 participants. The Conference Agenda is attached to this report.
A further highlight was the Arne Richter Award for Outstanding Young Scientists of the European Geoscience Union (2012) to Dr Encarnación Ruiz- Agudo, a Delta-Min Experienced Researcher Marie Curie Fellow.