To investigate the composition in greater detail, Raman spectroscopy line scans were performed on the same gastrolith crosssections (Fig. 6A and B). Spectroscopy data are shown in Fig. 6A. Integration of some of the peaks (Fig. 6B) reveals a trend in composition going from the later deposited external layers (spectrum nr.3) to the earlier deposited inner layers (spectrum nr. 80). Analysis of the Raman spectra suggest that the P-rich regions are in fact containing a predominantly basic inorganic phosphate (PO43), as can be derived from the positions of the main signal at 960 cm1 (m1 PO43), as well as the presence of less intense vibrations at 440 cm1 (m2 PO4 3) and 600 cm1 (m4 PO4 3) (Fig. 6B and C). Indeed the distribution of the PO4 3 follows very well the P-distribution in EDS, where the most intense signals are coinciding with a decrease in inorganic carbonate, and PO43 levels are varying between 1 and 30 mol% (see Table 1). Furthermore, the broadness of the main phosphate (width m1 PO43 30 cm1) and carbonate (width m1
CO32 28 cm1) signals confirms the amorphous nature.
This can also be concluded from synchrotron wide-angle X-ray scattering
(WAXS) data of the embedded samples (Fig. 7). Here, independent
of the position in the gastrolith, all mineral shows a very
similar, amorphous diffraction pattern consisting of two broad
Gaussian-shaped bands (AMO1 and AMO2) with maxima at around
22 nm1 and 31 nm1. However, after careful fitting the bands
with Gaussians we can observe that the position of the fitted peak
at 22 nm1 shifts to lower values at the same places in the gastrolith
where EDS shows a higher P-concentration (Fig. 7B, left axis
and Fig. 7C, right axis). At the same time the ratio in area between
AMO2 and AMO1 moves toward lower values (Fig. 7B, right axis).
This behavior, where the shape of the WAXS pattern depends on
the P-concentration, can be explained by comparing the WAXS