Ca-Doped Strontianite–Calcite Hybrid Micropillar Arrays Formed via Oriented Dissolution and Heteroepitaxial Growth on Calcite

Abstract

Even though heteroepitaxial growth of one-dimensional microstructures or three-dimensional islands has been realized via strategies such as chemical vapor deposition, molecular beam epitaxy, and solution phase, researchers have not succeeded in the formation of one-dimensional hybrid microstructures via the combination of heteroepitaxial growth and oriented dissolution up to now. In this work, well ordered single crystalline calcium-doped strontianite−calcite hybrid micropillar arrays with the long axis along the [001] direction of the two phases were formed on calcite (104) substrates via heteroepitaxial growth of strontianite and oriented dissolution of calcite in aqueous solution for the first time. Energydispersive spectroscopy analysis shows that the micropillars are composed of strontianite occluded with about 12 mol % of Ca. During the growth process of strontianite, calcite cores formed in the inner side of the strontianite micropillars at the base stage via the oriented dissolution. The lattices of the strontianite and calcite match very well at the interface of calcite and strontianite. A novel formation mechanism is proposed for the formation of Ca-doped strontianite−calcite hybrid micropillars via the synergetic process of the oriented dissolution of one carbonate mineral and the heteroepitaxial growth of another carbonate mineral in Volmer−Weber growth mode with a small lattice misfit. Ca-doped strontianite micropillar arrays might grow epitaxially on the calcite (001) and (104) planes with small misfits.

Controls of Microstructure and Chemical Reactivity on the Replacement of Limestone by Fluorite Studied Using Spatially Resolved Small Angle X-ray and Neutron Scattering

“…The newly formed fluorite crystals in the low porosity CM limestone exhibit acicular growth, but the individual crystals appear to be randomly oriented (Figure ). In the prior study of Pedrosa et al, in the case of pure calcite replacement, formation of oriented, sheet-acicular crystals was reported, similar to other reports of the replacement of calcite by strontianite . In our experiments, the fluorite needles are randomly oriented, which could be caused by the space that is available for precipitation within the limestone.…”

“…In the prior study of Pedrosa et al, 47 in the case of pure calcite replacement, formation of oriented, sheet-acicular crystals was reported, similar to other reports of the replacement of calcite by strontianite. 41 In our experiments, the fluorite needles are randomly oriented, which could be caused by the space that is available for precipitation within the limestone. Possibly, this acicular orientation is caused by the replacement, which proceeds faster along the grain boundaries than within the grains (Figure 11b and Figure S17 of Supporting Information).…”

“…A number of studies have considered porosity changes during dissolution reprecipitation reactions, ,,− , and furthermore, several studies have investigated the effect of pre-existing microstructure on replacement reactions. ,,, Naturally reacted samples obtained during field studies on the other hand, contain significant microstructure − but are often too complex to evaluate completely as the conditions under which they formed are, at best, uncertain. We have, therefore, performed controlled laboratory replacement experiments on samples from three rock formations, two limestones and a dolostone, to study the relations between microstructure and replacement extent and speed.…”

Pb2+–Calcite Interactions under Far-from-Equilibrium Conditions: Formation of Micropyramids and Pseudomorphic Growth of Cerussite

“…The micropyramids have a morphology similar to calcite growth hillocks. ,, Since the reaction solution was acidic and undersaturated with respect to all CaCO 3 phases, it is unlikely that the micropyramids were growth hillocks formed by precipitation of dissolved carbonate species. Preserved dissolution features containing calcite nano shoots/rods have been observed during calcite dissolution in the presence of formamide, NH 4 I, and (NH 4 ) 2 SO 4 , − but formation of calcite micropyramids in metal ion bearing solutions has not been reported before. The shape and orientation of the basal planes of the micropyramids are similar to those of the etch pits (Figure b, red and black boxes).…”

Patterned Calcite Microneedle Arrays formed on Mollusk Shells via Oriented Dissolution and Their Application for Transdermal and Intracellular Delivery

“…[66] Our previous studies have shown that microneedles along the c-axis may form via oriented dissolution of geological calcite and biogenic calcite minerals in the presence of additive molecules. [67][68][69][70][71][72] Considering the very good biological compatibility, high mechanical properties, ease of synthesis, and high surface area, the calcite microneedle arrays formed on calcite via oriented dissolution are supposed to be good substrates for transdermal delivery. Oriented dissolution is a very convenient and simple method to prepare microneedle arrays, in comparison to the complicated synthetic methods for microneedle-based transdermal delivery such as lithography, electroplating, and wet and dry etching.…”