Effects of fluid flow and solute transport on anorthite dissolution rates in heterogeneous pore networks

The effect of transport control on anorthite mineral dissolution rates was investigated in variably connected pore networks at pH 3, 4, and 5 using microfluidic devices with pore networks laser ablated directly into a polished mineral wafer. Dead-end pores create zones where the solute transport is slow and concentrations of dissolution products increase relative to connected channels in the pore network. Anorthite dissolution rates calculated from effluent Ca²⁺ fluxes out of microfluidic devices with full pore connectivity were similar to dissolution rates measured with other experimental approaches and ranged from 1.78 × 10^-8 to 1.37 × 10^-9 mol anorthite m^-2 s^-1 for fluid residence times of ∼3, ∼6, and ∼12 minutes and fluid pH of 3, 4, and 5. When connected pathways within the pore network were converted to dead-end pores in the microfluidic device design, dissolution rates decreased at both pH 3 and 5 when Ca flux was normalized to the total mineral surface area in contact with the pore network. We use Damköhler and Peclet numbers to demonstrate the varying transport- and reaction-limited conditions in connected and dead-end pores and show that transport in dead-end pores is diffusion dominated. This study illustrates that accessible mineral surface area and pore network connectivity are important controls on mineral dissolution rates in systems with pore network heterogeneity that creates high percentages of dead-end pores.