Vaterite: a mineralization product of the hard tissues of a marine organism (Ascidiacea).

X-ray diffraction and infrared absorption spectra show that the spicules of the common tropical ascidian, Herdmania momus, are mineralized with vaterite. These are the first strictly marine organisms known to normally precipitate vaterite. The biomineralization of vaterite may constitute another link between the urochordates and vertebrates. The vaterite of ascidian spicules immersed in natural seawater remains mineralogically unchanged for 1 year, which indicates that vaterite may be preserved transiently in marine sediments.