Fig. 5. FTIR demonstrates that shell windows have a high proportion of c-axis aligned aragonite crystals compared to opaque regions.

Here we plot three characteristic spectra taken from polished fragments of shell. Peak location corresponds to crystallographic orientation^78,79; a-axis orientation (“A” in figure) is highlighted in salmon, b-axis-orientation (“B”) in pale purple, and c-axis-orientation (“C”) in pink. B- and c-oriented crystals produce very similar spectra, but the peak located between 853–877 cm⁻¹ is unique to a-oriented crystals. a Reflectance from an opaque region of the shell shows a high proportion of a-oriented crystals. b Reflectance from a semi-opaque, semi-window region of the shell shows a medium proportion of a-oriented crystals. c Reflectance from a window region of the shell shows a low proportion of a-oriented crystals. Because the c- and b-axes of aragonite are so similar, it is difficult to distinguish which of these higher refractive indices is oriented in the Z direction. According to the literature, the c-axis is generally thought to be parallel to the long axis of aragonite prisms⁸³; therefore, we make the same conclusion. Whether it is the c-axis, b-axis, or both, our results do not materially change. Source Data for Fig. 5 can be found in FTIRMeasurements_29April2022.zip (Supplementary Data 1.zip).