AN X-RAY-ABSORPTION SPECTROSCOPY STUDY OF THE STRUCTURE AND TRANSFORMATION OF AMORPHOUS CALCIUM-CARBONATE FROM PLANT CYSTOLITHS

Cystoliths are intracellular calcified bodies which are found in great numbers in the leaves of many higher plants such as Ficus retusa. The mineral part of these deposits is amorphous calcium carbonate, which transforms to calcite only when moistened. We have followed this trans formation by using X-ray spectroscopy by measuring the local atomic st ructure around the calcium of the isolated dry cystoliths and after mo istening them with water. The production and maintenance of the amorph ous phase is clearly under biological control. The cystoliths may act as a pH-stat which neutralizes hydroxide ions. Potentially cytotoxic c ations also accumulate in the cystoliths. Rapid precipitation of calci um carbonate into the organic matrix could favour the amorphous phase, which may be maintained by low concentrations of magnesium and phosph ate, which are inhibitors of the nucleation of crystalline phases.