The {010} form of polar brushite (CaHPO4·2H2O) deposits as an epi-growing crystal on a non-polar {010} substrate of gypsum (CaSO4·2H2O)

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Abstract
The mechanisms of crystal growth from solution are more complicated than those from vapor or melt growth. These mechanisms are important in biomineralization (bones, teeth, shells) and in the characterization of optically non-linear crystals, perfectly crystallized proteins, nucleic acids and complex viruses. This motivated studies on new basic phenomena, including the crystal interfaces of biological macromolecules and those of low-solubility inorganic salts. Generalization of the Gibbs–Thomson effect has been questioned when describing isotropic (circular) or anisotropic (polygonized) spiral steps spreading onto a crystal face. This work focuses on the brushite crystal (CaHPO4·2H2O) since it can grow freely or as a deposit on a gypsum (CaSO4·2H2O) substrate. Brushite looks like a 2D crystal built by a flat {010} form limited by {h0l} polar and thin faces running as straight steps. Moreover, it is worth investigating the {010} gypsum to {010} brushite transformation since mineralogy and geochemistry play an important role in the crystal growth. Here, we study the freely growing brushite and, subsequently, its {010} form arising from gypsum. The epitaxy {010} gypsum/{010} brushite is also examined because brushite twins are involved. A new way of thinking about the epi-relations is proposed, along with more rigorous constraints; this new approach emphasizes the physical importance of the adhesion energy connected to every epitaxy.