Epitactic Growth during the Fluid-Driven Aragonite-to-Calcite Transformation

Abstract

Aragonite and calcite (CaCO3) are the most abundant polymorphs of calcium carbonate in the Earth’s crust and are relevant industrial minerals. The factors that control the solid-state transformation of aragonite, the high-pressure polymorph, into calcite, the stable polymorph at ambient P-T conditions, have been thoroughly studied. However, how this transformation proceeds when it occurs through a rapid dissolution–precipitation mechanism, the pathway most prevalent at Earth’s surface conditions, remains poorly understood. In this work, we conduct hydrothermal interaction experiments between the common (001) and (110) aragonite surfaces and either deionized water or a 0.5 M Na2CO3 solution at 220 °C and study the characteristics of the calcite overgrowth replacing the parent aragonite. Calcite layers initially consist of isolated, oriented 3D crystals, which later grow to coalesce, forming porous Volmer–Weber layers. We identify and structurally evaluate the following epitaxies: (001)Arg∥(00.1)Cal, with a low lattice misfit (∼0.6–8%), and a novel non-c-axis-parallel relationship (110)Arg∥(10.4)Cal, which challenges c-axis-centric existing models. Individual calcite crystal habits evolve from rhombohedral to elongated as a result of substrate-induced twinning and anisotropic growth, replicating microtextures observed in natural blueschist metamorphic rocks and validating laboratory analogues. We discuss the possible impact of the described crystallographic controls on the kinetics of this fluid-driven mineral transformation, highlighting its significance as a fundamental reaction in Earth sciences with implications for carbonate diagenesis and the design of new materials.