Phase transformation of Mg-calcite to aragonite in active-forming hot spring travertines

Abstract

A travertine specimen collected from the western part of Yunnan Province of China was subjected to microstructural analysis by powder X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. A new formation mechanism was proposed whereby polycrystalline rhombohedral particles of magnesium-containing calcite underwent a phase transformation into sheaf-like clusters of aragonite microrods. It is proposed that a high concentration of magnesium ions and embedded biological matter poisoned the growth of calcite and therefore instigated the phase transformation of the core of the rhombohedral calcite particles to an aragonite phase with a higher crystallinity. The single crystalline aragonite microrods with a higher density than the Mg-calcite nanocrystallites grew at the expense of the latter to generate sheaf-like clusters. This newly discovered formation mechanism is expected to enhance previous knowledge on this geologically important phase transformation from a morphology point of view.2

Droplet-based in situ X-ray absorption spectroscopy cell for studying crystallization processes at the tender X-ray energy range

“…38 In this work, aragonite formation was not detected at any stage of the crystallization process because the experiments were done in big droplets, at room temperature and without any aragonitepromoting additive such as magnesium ions. This is consistent with the literature, where at room temperature aragonite formation has been observed to occur either in connement $25 nm pores 39,40 or in solutions containing inorganic additives such as magnesium ions [41][42][43][44][45][46][47][48] or organic additives/solvents. [49][50][51] Carbon dioxide has a strong inuence on the pH of aqueous solutions because it readily dissolves in water to form carbonic acid, which dissociates to bicarbonate and carbonate ions respectively (eqn (1)).…”

A study on the influence of sodium carbonate concentration on the synthesis of high Mg calcites

“…No phase transformation occurred even when the hydrothermal time was extended to 5 days in our system, unlike the phase transformation from magnesium calcite to aragonite in a travertine crust specimen. 44 We propose that the hydrothermal process at 150 °C for 24 hours is appropriate for the synthesis of well crystallized high Mg calcites in this work. The hydrothermal process for the synthesis of high Mg calcites ]/[Ca 2+ ] and [Mg 2+ ]/[Ca 2+ ] were changed in this work.…”

Study on preparation and structure of chrysanthemum‐shaped micron calcium carbonate based on inverse microemulsion

“…3.1. Morphology: In the process of experiment, we found that different preparation methods, even a small change in the type and amount of additives or a slight change in reaction conditions would cause a huge change in crystal morphology and particle shape [30][31][32][33].…”