X-Ray absorption spectroscopy studies on the structure of a biogenic "amorphous" calcium carbonate phase

Amorphous calcium carbonate is formed by a surprisingly large number of organisms, bearing in mind how unstable it is. Organisms use amorphous calcium carbonate as temporary storage sites for ions, as precursor phases that transform into more stable crystalline calcium carbonate polymorphs, or in a stabilized form for mechanical purposes. Here one example is examined that fulfils the latter function; the so-called antler shaped spicules formed by the ascidian Pyura pachydermatina. These spicules are composed of amorphous calcium carbonate containing 16% (w/w) water and 14 mole% phosphate. A detailed X-ray absorption spectroscopy (XAS) study of the calcium K-edge in the spicules shows a first co-ordination shell with seven/eight oxygen atoms, and a second co-ordination shell with four/five carbon atoms. The best fit was obtained using monohydrocalcite as model, and is consistent with a slightly expanded hydrated structure. It is noteworthy that the XAS spectrum of these spicules is quite different from that reported previously for the amorphous calcium carbonate of plant cystoliths. This raises the intriguing possibility that the biogenic amorphous phases differ structurally from each other, and that the differences can account for their diverse modes of formation and function.