which correspond to the darker layers in transmission light microscopy (see also Figs. 1 and 2 D). Together with the loose packing of spherical features in the middle of the gastrolith, where P-content seems to be highest (Fig. 4), this indicates a correlation between the phosphorous content and nanosphere stability against aggregation.
However, also the age of the layers seems to play a role here, where in the later deposited external layers the spheres are loosely packed, although here P-contents are very low.