An X-ray absorption spectroscopy study of the structure and transformation of amorphous calcium carbonate from plant cystoliths

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Abstract
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 transformation by using X-ray spectroscopy by measuring the local atomic structure around the calcium of the isolated dry cystoliths and after moistening them with water. The production and maintenance of the amorphous phase is clearly under biological control. The cystoliths may act as a pH-stat which neutralizes hydroxide ions. Potentially cytotoxic cations also accumulate in the cystoliths. Rapid precipitation of calcium carbonate into the organic matrix could favour the amorphous phase, which may be maintained by low concentrations of magnesium and phosphate, which are inhibitors of the nucleation of crystalline phases.