Self-Generated Electrokinetic Fluid Flows during Pseudomorphic Mineral Replacement Reactions

Abstract

Pseudomorphic mineral replacement reactions involve one mineral phase replacing another, while preserving the original mineral's size and texture. Macroscopically, these transformations are driven by system-wide equilibration through dissolution and precipitation reactions. It is unclear, however, how replacement occurs on the molecular scale and what role dissolved ion transport plays. Here, we develop a new quantitative framework to explain the pseudomorphic replacement of KBr crystal in a saturated KCl solution through a combination of microscopic, spectroscopic, and modeling techniques. Our observations reveal that pseudomorphic mineral replacement (pMRR) is transport-controlled for this system and that convective fluid flows, caused by diffusioosmosis, play a key role in the ion transport process across the reaction-induced pores in the product phase. Our findings have important implications for understanding mineral transformations in natural environments and suggest that replacement could be exploited in commercial and laboratory applications.

Reaction-induced porosity fingering: Replacement dynamic and porosity evolution in the KBr-KCl system

“…The high-resolution 3D dataset of the temporal evolution of KBr-KCl replacement reaction with X-CT allows key observations in order to refine our understanding about how much porosity develops during the replacement, how porosity is distributed over time, and how the reaction front progresses into the crystal. These quantitative observations complement previous works (Kar et al, 2016;Raufaste et al, 2010) and allow us to better understand the dynamics of the replacement propagation and associated mechanisms, such as volume change, porosity geometry and transport regime. By comparing two sets of experiments that mimic two different environments for fluid-rock interactions (single event versus intermittent flow), the phenomenon we describe and quantify can ultimately be transferred to other natural systems.…”

“…in Kondratiuk et al, 2015). The KBr-KCl replacement exhibits an elongated anisotropic porosity that controls the replacement propagation (Kar et al, 2016;Raufaste et al, 2010). Fluid flow can extend pores to create the elongated structures (Kar et al, 2015;Raufaste et al, 2010), which are similar to finger pipes observed in nature during dissolution (Szymczak and Ladd, 2009).…”

“…We use a highly soluble, cubic system: the KBr-KCl replacement (Kar et al, 2016;Putnis and Putnis, 2007;Putnis and Mezger, 2004;Raufaste et al, 2010;Spruzeniece et al, 2017). This system is well suited for temporal 3D imaging of replacement; it has a fast reaction time (hours) with visible porosity evolution and a stable end product.…”

The replacement of a carbonate rock by fluorite: Kinetics and microstructure

“…is consistent with the determined value of E a and the Avrami exponential n-value, and emphasizes the importance of the porosity generation in mineral replacement processes. The study provides data for future work on determining the mechanism of ion transport through micropores, notably the recent discussions on the role of charge gradients in small pores in enhancing transport by diffusioosmosis (Kar et al, 2016).…”

Instabilities and finger formation in replacement fronts driven by an oversaturated solution

“…The stability of these fronts [ Kondratiuk et al , ] will be investigated in future work. Interestingly, recent studies [ Raufaste et al , ; Kar et al , ] report fingering at a microscopic scale (∼100 μm), presumably connected with self‐generated convective flows by the process of diffusion osmosis [ Kar et al , ; Ajdari and Bocquet , ].…”