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

The mechanisms of crystal growth from solution are more complicated than those from vapor or melt growth. These mechanisms are important in bio-mineralization (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.