Crystal Growth and Experimental Aquatic Geochemistry
Manuel Prieto Rubio, Professor (Dept.
of Geology. Univ. Oviedo)
Lourdes Fernández González, Lecturer (Dept. of Crystallography, Univ. Madrid)
Ángeles Fernández González, Assistant Professor (Dept. of Geology, Univ. Oviedo)
Amalia Jiménez, Assistant Professor (Dept. of Geology, Univ. Oviedo)
Carlos Pina Martínez, Marie Curie Fellowship (Institut für Mineralogie, Univ. Münster)
Jose Manuel Astilleros, Postgraduate Student (Dept. of Crystallography, Univ. Madrid)
Pablo Cubillas González, Postgraduate Student (Dept. of Geology, Univ. Oviedo)
Ángel José Andara, Postgraduate Student (Dept. of Geology, Univ. Oviedo)
Research Training Network
in Natural and Industrial Processes
Our group is involved in a project supported by the European Commission which is focused to the understanding of the dissolution and precipitation rates of solid solutions. Six different groups participate in this project:
|Crystallisation in solid solution /aqueous solution (SS-AS) systems|
From some years ago, our group is involved in the study of the crystallisation behaviour of solid solutions of different carbonates and sulphates. The goal of this research is to understand ion partitioning in mineral-aqueous solution interactions. Particular attention has been paid to the partitioning of divalent metals into calcite, which is of interest in order to study water-rock interaction that involve carbonate minerals and in environmental assessment of metal ion behaviour. Crystallisation of solid solutions represents a potentially significant metal-scavenging process in aqueous phases. Divalent metals as Cd, Zn, Co, Ni, Mn, Pb, etc.. can be removed from the aqueous phase by precipitation of (M,Ca)CO3 solid solutions or by growing an adherent layer of solid solution on the surface of carbonate minerals. Other elements of high toxicity, like Cr(VI), can be removed by co-crystallization with different sulphates, in the form of M(SO4,CrO4) solid solutions. However, in spite of the numerous studies on this matter, the fundamental controls on ion partitioning in SS-AS systems during crystal growth remain poorly understood. While a thermodynamic approach is very useful, the importance of studying the crystallisation behaviour in a kinetic or mechanistic framework is becoming more and more apparent.
The previous scenery guides the present interests of our group that can be particularised according to the following topics:
The main difficulty in establishing the equilibrium behaviour in SS-AS systems is the absence of thermodynamic data about the mixing properties for many solid solutions. The experimental determination of mixing properties is very time consuming and has many sources of uncertainty. Moreover, in many experimental studies there are potential problems of achieving equilibrium between the solid and the aqueous phase. In two recent papers we have investigate the possibility of determining the complete set of mixing properties from molecular principles calculations. The basic idea is to relate the microscopic details of atomic interactions to site occupancies and macroscopic thermodynamic parameters. As long as the computational parameters (potential sets, etc..) are carefully chosen, the accuracy of the results can often compete with experimental data. In addition, modelling ordered and random distributions can give an insight into ordering phenomena within the solid solution. Furthermore, to a certain degree, it is possible evaluate the conditions under which ordering may occur and the temperatures above which entropy effects are dominant.
We have calculated by this method the SS-AS phase diagram for the system (Ba,Sr)SO4 -H2O. Carbonate systems with an ordered phase (dolomite type) of intermediate composition - (Mg,Ca)CO3 , (Mn,Ca)CO3 , and (Mn,Cd)CO3 - are under study at this moment. This research is being carried out in collaboration with the Group on Mineral Surfaces of the University of Münster
Our group is specially interested in the study of the nucleation and growth of crystals of solid solutions from supersaturated fluids. In our studies, crystallisation experiments are usually carried out by the counter-diffusion of reactans through a column of porous silica hydrogel. In these experiments, nucleation takes place under controlled conditions of supersaturation and supersaturation rate. Not all natural processes occur at equilibrium, and this method allows the establishment of the non-equilibrium effect, i.e., the extent to which the reaction rate influences the distribution of substituting ions between solid and aqueous phases. Under conditions of high supersaturation, the substituting ions tend to be laid down in a ratio which deviates from the equilibrium distribution. Moreover, kinetics effects lead to the persistent occurrence of metastable compositions in solid solutions with miscibility gaps or ordered intermediate phases.
At present we are studying by this method the nucleation behaviour of solid solutions of carbonates (M,Ca)CO3 (M = Ba, Cd, Co, Mg, Mn, Ni, Pb, and Sr) and sulphates (Ba,Sr)SO4, (Ba, Pb)SO4, Ba(SO4,CrO4), and Ba(SO4, eO4).
During the growth process, substituting ions are not incorporated into the solid in the same stoichiometric proportion as in the aqueous phase. Therefore, crystal and aqueous solution compositions tend to vary as growth proceeds and this evolution is recorded in the crystal as a concentric compositional pattern. Concentric zoning delineates former growth surfaces and provides a record of the crystal morphology and of the temporal changes in the physicochemical conditions in existence during crystallisation. From these patterns, the growth history of the crystals can be observed retrospectively, and those factors which are influenced by the evolution in the bulk fluid composition can be separated from those which promote development of oscillatory zoning. We have demonstrated that oscillatory zoning in these experiments is not associated with an oscillating bulk fluid composition but with local fluctuations inherent in the growth process under certain conditions. Oscillatory zoning could be explained as being due to non-linearities caused by coupling of growth parameters such that chemical variations arise spontaneously: oscillatory-zoned crystals represent the material record of a "chemical oscillator".
At present we are performing a dynamic model to account for the reaction paths during the growth process and to establish the limiting conditions under which oscillatory zoning becomes apparent. The study is being carried out for the systems (Cd,Ca)CO3, (Mn,Ca)CO3 and (Zn,Mn)CO3 and (Mn,Cd,Ca)CO3. This research is being carried out in collaboration with the Münster Group on Mineral Surfaces
The study of SS-AS systems has environmental implications. The sorption (and subsequent solid solution formation) of metals on the surface of carbonate minerals represents a potentially significant metal-removing process in aquatic, marine, and groundwater environments. In order to evaluate the effectiveness of this process, we are carrying out experiments to study the retardation in the transport of dissolved divalent metals through porous media consisting of grains of calcite (or barite) embedded in a matrix of silica gel. In these experiments, sorption isotherms can be determined by comparing the metal diffusivity through the studied porous medium to the diffusivity through an equivalent unreactive medium. After diffusion the mineral grains can be easily released from the gel matrix in order to study their surface. When sorption occurs via surface precipitation of a metal bearing solid the method allows characterising the nature of the sorbed entities. We are studying by this method the transport of Cd2+, CrO42-, and Pb2+.
Atomic Force Microscopy is a powerful tool to follow crystal growth processes at a molecular scale. "In situ" studies of the growth behaviour of calcite surfaces from aqueous solutions containing divalent metals have been carried out in collaboration with the Münster Group on Mineral Surfaces. This technique has also been used to study solvent-mediated polimorphic transformations and other coupled dissolution/precipitation reactions in aqueous environments.
List of publications related with these topics.
16 de Septiembre de 2002