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GEOCHEMISTRY OF TRACE ELEMENTS DURING INTERACTION OF WATER A WITH MINERALS OF CARMENELLIS GRANITE AND ITS RELEVANCE TO HDR SYSTEMS

Objective

TO STUDY THE REACTIONS BETWEEN WATER AND ROCK IN ORDER TO OBTAIN A BETTER UNDERSTANDING OF THE REACTIONS THAT HAPPEN IN A HDR SYSTEM.
Whenever water is brought in contact with rock forming minerals dissolution and/or precipitation reactions occur at the solid solution interface. The understanding of these reactions is of great importance for the effective exploitation of a hot dry rock (HDR) geothermal system.

The major part of the experimental studies on the dissolution mechanism of minerals are based on feldspar leaching since this is the most important group of rock forming silicate minerals. Whenever feldspars interact with an aqueous solution an enhancement of some elemental concentrations in solution is observed together with a weathered mineral surface. Studies of the solution after weathering resulted in the leached or diffusion layer hypothesis where one assumes a nonstoichiometric dissolution step based on the frequently observed parabolic dissolution kinetics. This model predicts an altered layer of a few 10s of angstroms through which reactants and products of weathering must diffuse. In a second mechanism, based on measurements of the solid surface, a surface reaction is assumed, which implies a stoichiometric dissolution step. Recent studies emphasize the important role of high energy surface sites such as outcrops, layer edges and microcracks. The diffusion layer model proposed by Chou and Wollast (1985) accounts for both the observed solution and solid surfacechemistry and takes the contribution of surface defects into consideration.

The latter model predicts the formation of an hydrogen feldspar as a result of an instantaneous exchange reaction between alkali ions and hydrogen (hydronium (H3O) ions), followed by a rapid build up of a potassium, sodium depleted layer enriched in silicon (for acidic conditions) and a slow diffusion of ions from the fresh feldspar through the residual layer. This hydrated leached layer has, however, never really been observed.
The aim of this study is therefore to look for the presence of the residual layer and to characterize it if present.

The altered surface of sanidine grains leached in acid solutions of pH=1 were examined by secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and electron probe X-ray microanalysis (EPMA).

A layer depleted in sodium, potassium and aluminium was observed at the surface of sanidine grains leached in 0.1 normal hydrogen chloride (or deuterium chloride). Its thickness increased with increasing reaction times and/or temperatures when the silicon gradient at the interface was lowered by adding pure silicon dioxide powder to the solution. The observation of a pure hydrated silicon dioxide surface when feldspar is leached in an acid environment, was in qualitative agreement with the mechanism proposed by Chou and Wollast (1985).
There is some evidence for the existence of a hydrogen feldspar (H2Al2Si6 O16) from the FT-IR measurements, although the evidence was limited to highly leached grains. The calculated diffusion coefficients from the potassium/silicon depth profiles have the same order of magnitude as values previously reported. On the other hand, an increasing amount of etchpits is observed with the SEM which indicates that dissolution actually takes place at these surface defects. It has already been suggested that migration of molecular water into the surface is a key step in the dissolution mechanism of silicate minerals. The latter is confirmed by the large amount of adsorbed water observed in the FT-IR spectra.
WITHIN THE FRAMEWORK OF THE HDR PROJECTS IN THE CARNMENELLIS GRANITE (CORNWALL, U.K.) A NUMBER OF EXPERIMENTS AND ANALYSES ARE NECESSARY FOR THE UNDERSTANDING AND THE FUTURE DEVELOPMENT AND EXPLOITATION OF SUCH ENERGY SOURCES. BECAUSE OF THE EXPERIENCE IN MULTI-ELEMENTAL TRACE ANALYTICAL TECHNIQUES PRESENT IN THE UNIVERSITY OF ANTWERP, THIS GROUP WILL FOCUS ITS RESEARCH ON THE LOCATION, ANALYSIS AND BEHAVIOUR OF TRACE ELEMENTS DURING WATER-ROCK INTERACTIONS. SEE ALSO CONTRACTS 0002/B, 0079/B, 0057/UK AND 0010/F.
ADVANCEMENT:
THIS CONTRACT IS A CONTINUATION OF CONTRACT 0001/B

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

UNIVERSITAIRE INSTELLING ANTWERPEN - UNIVERSITEIT ANTWERPEN
Address
Universiteitsplein 1
2610 Wilrijk (Antwerpen)
Belgium