Effects of halogen in chemical exchange and porosity evolution during feldspar-fluid reaction interface

Gan Duan1, Joël Brugger1, Alkiviadis Kontonikas-Charos2, Rahul Ram1, Barbara Etschmann1 and Paul Guagliardo3

1School of Earth, Atmosphere and Environment, Monash University, Clayton, Victoria 3800, Australia,2School of Chemical Engineering and Advanced Materials, The University of Adelaide, Adelaide, SA 5005, Australia,3Centre for Microscopy, Characterisation, and Analysis, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

Interface coupled dissolution-reprecipitation (ICDR), through which dissolution of the parent phase produces is coupled to the precipitation of the product phase(s), is a widespread mechanism for this fluid-driven mineral replacement. The mode(s) of ion transport in ICDR and the nature of the mineral-fluid interface at the micron- to nanometer- scale are fundamental controls on the coupling between mass transfer, fluid flow, porosity creation/destruction, and mineral replacement that is responsible for large-scale fluid-rock interaction.

In this work, the effect of halogen (chlorine and fluorine) in alkali exchange during the mineral fluid reaction interface was investigated on a model system wherein sanidine (K,Na)AlSi3O8) was reacted with a combination of NaCl, NaF or FeF2 and 18O-enriched solutions at 600 °C and 2 kbar, leading to replacement by albite and/or K-feldspar. Nanoscale secondary ion mass spectrometry (Nano-SIMS) of oxygen isotopes (16O vs. 18O) in the parent and product phases at high-spatial resolution, showed that Al-O and Si-O bonds in the sanidine tectosilicate framework were broken during replacement. Transmission electron microscopy (TEM) of “foils” cut across the reaction boundary indicated a sharp reaction boundary and interfacial zone between reactant and products, consistent with the ICDR mechanism. Based on its morphology (distinct stress-induced S-shape distortion), the results suggest that the interfacial zones consist of an amorphous hydrosilicate colloid or gel rather than a fluid or solid phase. The ion diffusion occurs horizontally in this dynamic interfacial layer, with exchanges between interface and bulk solution occurring along cracks/defects in the product; this is further indicated by the absence of connected porosity in the bulk newly precipitated phases. Compared to chlorine-bearing solutions, in fluorine-bearing solutions, element diffusion in the interfacial zone is likely enhanced by the formation of Na-Si-Al-fluoride complexes, which results in more extensive reaction progress. The presence of fluoride also influenced the fate of minor amounts of Ti in subsequent mineral replacement.

These results reveal that the nature of the reaction interface depends on both the nature of the mineral and on solution chemistry; this directly affects porosity evolution and chemical exchanges between the reaction front to the bulk solution. Altogether these results can further help explain why the sodic and potassic alteration associated with metal and fluid transport in some of the world’s largest ore deposits can develop extensively and access the metals locking in silicate mineral assemblages.


Biography

I am PhD student from Monash Uni. My current study is investigate the mineral fluid reaction process that happened in kilometre to nanometre- scale in nature. I investigate alteration reactions (e.g., albitization, K-feldspartization) through both field, laboratory and thermodynamic modelling work.

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