Abstract
The co-precipitation of sulphate minerals such as celestine and barite is widely studied because their formation is ubiquitous in natural and anthropogenic systems. Co-precipitation in porous media results in crystallization of solid solutions yielding characteristics such as oscillatory zoning that are rarely observed in bulk solution or in batch experiments. In the past, the precipitation of compositionally-zoned (Ba,Sr)SO4 crystals was observed post-mortem in macroscopic silica gel counter-diffusion experiments. Their formation was originally explained by the difference in the solubility products of the end-members combined with diffusion-limited transport of solutes to the mineral-fluid interface, while a later study favored the idea of kinetically controlled reactions. With recent advances combining in-operando microfluidic experiments and reactive transport modelling, it is now possible to verify hypotheses on the driving forces of transport-coupled geochemical processes. We developed a “lab on a chip” experiment that enabled the systematic study of the nucleation and growth of oscillatory-zoned (Ba,Sr)SO4 crystals in a microfluidic reactor. The compositions of the solid solutions were determined by in-situ Raman spectroscopy. Our investigation shows (1) that the composition of the nucleating phases can be approximated using classical nucleation theory, (2) that the oscillatory zoning is not solely controlled by the limited diffusional transport of solutes, and (3) that nucleation kinetics plays a major role in the switch between different stoichiometric compositions. The zoning phenomena is governed by the complex interplay between the diffusion of reactants and the crystallization kinetics as well as other factors, e.g. surface tension and lattice mismatch.
Highlights
MethodsExperimental setup and procedure.The experimental setup consists of a microfluidic reactor that is connected to pumps and monitored by optical microscopy and Raman spectroscopy (Fig. 1a)
The maxima of these functions lie at XBa = 0.99. This is the solid composition with respect to which supersaturation is highest and it corresponds to true thermodynamic equilibrium with the specific aqueous composition
We developed a lab on a chip experiment that enabled the systematic in-situ assessment of oscillatory zoned crystals of (Ba,Sr)SO4 in a confined volume
Summary
Experimental setup and procedure.The experimental setup consists of a microfluidic reactor that is connected to pumps and monitored by optical microscopy and Raman spectroscopy (Fig. 1a). The microfluidic reactor is composed of two adjacent supply channels and 50 growth chambers (Fig. 1b). A narrow channel of 10 μm by 10 μm connects the supply channels to the growth chambers. The narrow connections between the supply channels and the growth chambers enable a diffusion dominated transport regime in the growth chamber. The barrier structures (Fig. 1d) consist of an array of rectangular pillars (of 7 μm length and 2.37 μm width) and distanced by 0.6 μm placed in the middle of the chamber. They maintain mechanical stability of the chamber and serve as substrate to initiate the nucleation process[22]. The microfluidic reactor was made out of PDMS (Polydimethylsiloxane) and closed with a glass cover
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