Many challenges related to carbon-dioxide (\documentclass[12pt]{minimal}
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				\begin{document}$$\hbox {CO}_2$$\end{document}CO2) sequestration in subsurface rock are linked to the injection of fluids through induced or existing fracture networks and how these fluids are altered through geochemical interactions. Here, we demonstrate that fluid mixing and carbonate mineral distributions in fractures are controlled by gravity-driven chemical dynamics. Using optical imaging and numerical simulations, we show that a density contrast between two miscible fluids causes the formation of a low-density fluid runlet that increases in areal extent as the fracture inclination decreases from 90\documentclass[12pt]{minimal}
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				\begin{document}$$^\circ$$\end{document}∘(vertical fracture plane) to 30\documentclass[12pt]{minimal}
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				\begin{document}$$^\circ$$\end{document}∘. The runlet is sustained over time and the stability of the runlet is controlled by the gravity-driven formation of 3D vortices that arise in a laminar flow regime. When homogeneous precipitation was induced, calcium carbonate covered the entire surface for horizontal fractures (0\documentclass[12pt]{minimal}
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				\begin{document}$$^\circ$$\end{document}∘). However, for fracture inclinations greater than 10\documentclass[12pt]{minimal}
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				\begin{document}$$^\circ$$\end{document}∘, the runlet formation limited the areal extent of the precipitation to less than 15% of the fracture surface. These insights suggest that the ability to sequester\documentclass[12pt]{minimal}
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				\begin{document}$$\hbox {CO}_2$$\end{document}CO2through mineralization along fractures will depend on the fracture orientation relative to gravity, with horizontal fractures more likely to seal uniformly.