(Ba,Sr)SO4 OSCILLATORY ZONING: A MICROFLUIDIC EXPERIMENT AND ADVANCED PORE SCALE MODELLING TO UNRAVEL THE FORMATION PROCESS

Abstract:

The co-precipitation of sulphate minerals is widely studied because their formation is ubiquitous in Nature and in anthropogenic systems. This has urged the development of various tools to rationalize the thermodynamics of solid solutions¹. However, kinetic effects can give rise to metastable solid solution and aqueous solution compositions. The implementation in numerical predictive tools still remains a challenging task and is accounted for by the consideration of the nucleation rate² and partial equilibrium³. During co-precipitation processes, kinetic effects can lead to spatial variations in the composition of the surrounding fluid, resulting in compositional heterogeneity in the mineral known as compositional zonation. Previously, the formation of compositionally-zoned (Ba,Sr)SO₄ crystals was triggered in silica gel counter-diffusion experiments³. The process was explained by the difference in the solubility products of the end-members combined with diffusion-limited transport of solutes to the mineral-fluid interface⁴ while other work promoted the idea of kinetically controlled reactions⁵. With emerging tools combining microfluidic experiments and pore scale modelling, it is possible to verify hypotheses on the driving forces of transport coupled geochemical processes^6,7. We developed a “lab on a chip” experiment that enabled the study of the nucleation and growth of oscillatory-zoned (Ba,Sr)SO₄ crystals in a microfluidic chamber. Our micromodel consists of two parallel supply channels interconnected by chambers. A Na₂SO₄ solution and a mixed solution of BaCl₂ and SrCl₂ were injected into the supply channels. The co-precipitation of (Ba,Sr)SO₄ was fostered by the counter diffusion of solutes in the chamber. The composition of the solid solution was determined by in-situ Raman spectroscopy as zoned crystals consisting of (Ba_0.5Sr_0.5)SO₄ and (Ba_0.05Sr_0.95)SO₄. Currently, we use pore scale modelling based on lattice Boltzmann methods to calculate the evolving fluid composition at the mineral-fluid interface. The outcome of our numerical investigations will be used to decipher the controlling mechanisms resulting in oscillatory zoning.

¹ Prieto 2009, ² Noguera et al. 2016, ³ Thien et al. 2014, ⁴ Putnis et al. 1992, ⁵ Pina & Putnis 2001, ⁶ Poonoosamy et al. 2019, ⁷ Prasianakis et al. 2020