Plant cystoliths: A complex functional biocomposite of four distinct silica and amorphous calcium carbonate phases

Plant cystoliths are mineralized objects that are formed by specialized cells in the leaves of certain plants. The main mineral component of cystoliths by volume is amorphous calcium carbonate (ACC) and the minor component is silica. We show that the silica stalk is formed first and is essential for ACC formation. Furthermore, the cystolith is shown to be composed of four distinct mineral phases with different chemical properties: an almost pure silica phase grades into a Mg-rich silica phase. This Mg-rich silica is overlaid by a relatively stable ACC phase. A bulky and less stable ACC phase encapsulates the first ACC phase. This architecture poses interesting questions about the role of Mg in the silica phase and suggests a strategy for ACC stabilization that takes advantage of a precise regulation of the mineral-growth microenvironment. The fantastic four: Cystoliths are mineralized objects that are mainly composed of amorphous calcium carbonate (ACC), which is found in the leaves of several plants. They have a unique composition and architecture of four distinct amorphous phases. A Mg-rich silica phase is essential for the formation of two distinct ACC phases. The inner ACC phase has inherently higher stability, presumably required by the sequential formation mechanism.