The control exerted by some invertebrates on the calcium carbonate polymorph produced is intriguing but not understood. Mytilus edulis shells, with the abrupt polymorph switch within their valves from an outer calcite to inner aragonite layer, are excellent examples of this phenomenon. Detailed crystallography of intact valves using Electron Backscatter Diffraction (EBSD) is considered in the context of quantitative chemical analyses by electron microprobe. Apart from the outer 40 μm, individual crystals that comprise the calcite layer of M. edulis differ from each other in terms of misorientation by less than 10°. Similar uniformity occurs in the inner aragonite layer with notable ‘mineral bridging’ between tablets of aragonite nacre. The first-formed aragonite laminae are sub-micron thickness and the subsequent laminae of uniform 1 μm thickness.

Variations in chemical composition through the two valves correspond in part with the distribution of the two polymorphs. Magnesium is present in notably higher concentrations within calcite than aragonite. However, the Mg2+ concentration in calcite is not uniform and increases with growth before decreasing at the polymorph switch. Sodium concentrations decrease steadily through the calcite layer. The aragonite layer is compositionally more uniform. Sulphur is not a good proxy for organic content in this system because it does not reflect the higher organic content of the aragonite. Sector zoning is not responsible for the element distribution seen here while differences in crystal size and association with organic components remain as possible explanations.

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