Fig 1.
Phorcus turbinatus shell organization.
(A) Overview of the entire shell with the transversal (upper) and longitudinal (lower) sections used in Experiment 1 and 3 and in Experiment 2, respectively. (B) SEM image of prismatic microstructure in the outer shell layer (OSL) and (C) nacre tablets in the inner shell layer (ISL). (D-E) Raman spectra of the OSL (upper) and ISL (lower) indicating that both layers share the same mineralogy (aragonite). (F) AFM amplitude and (G) phase scan of P. turbinatus OSL showing granular nanometric structures. (H+J) AFM amplitude images displaying the topographic profile of individual nacre platelets and the existence of nanometric granular subunits. (I+K) AFM phase images highlighting the inhomogeneity of the nacreous shell material. Dark grey areas identify the hard material component (aragonite) whereas lighter areas indicate the presence of softer and adhesive compounds (organic matrix). Red arrows show the location of the boundaries between platelets. (L) 3-D visualization of nacre structure.
Table 1.
Details of the heating experiments on P. turbinatus shell.
Fig 2.
Sketch showing the preparation of samples 3.4 and 3.5.
The first section was made before heating the shell. The surface (sample 3.4) was exposed to 310°C for 72 hours. After heating, the second cut was made, disclosing sample 3.5 which was not directly exposed during heating.
Fig 3.
TGA thermal curve of P. turbinatus OSL (black line), nacre (red line) and abiogenic aragonite powder (blue line).
For each curve, it is shown the sample weight loss (horizontal step) between 200 and 500°C.
Fig 4.
Shell pigment distribution and mineralogical response to heating.
(A) Confocal microscope image of an unheated shell section (sample 1.2). The red box (B) indicates the shell portion analyzed with CRM. (B) CRM map showing the heterogeneous distribution of the pigments in the OSL based on the presence/absence of the CRM R4 polyene peak. In the OSLW and ISL (dark blue) polyenes are absent whereas they are abundant in the OSLB (pink/yellow). (C-E) CRM maps of the distribution of the different mineralogical phases in the same specimen (sample 1.2) after exposure to elevated temperatures. Dark areas are shell portions formed of aragonite, whereas bright areas indicate the presence of calcite. OSLB = black portion of outer shell layer; OSLW = white portion of OSL; ISL = inner shell layer (nacre).
Table 2.
Progression of the aragonite-calcite transformation in shell sections of Experiment 1 (short thermal exposure).
OSLB = black portion of outer shell layer; OSLW = white portion of OSL; ISL = inner shell layer (nacre).
Table 3.
Mineralogical composition of shells exposed to medium and long thermal exposure (Experiment 2 and 3).
Fig 5.
SEM images of P. turbinatus microstructures after heating (Experiment 3).
Images in the first row refer to the OSL whereas the second row refers to the ISL. (E-F) Microstructures of the section directly exposed to 310°C. (G-H) Microstructures of the bulk shell heated at same temperature.
Fig 6.
AFM scans of the nanostructures after heating (Experiment 2).
Images in the first row refer to the OSL, whereas the second row represents the ISL.
Fig 7.
AFM scans of the nanostructures after long thermal exposure (Experiment 3).
Images of the first row refer to the OSL, whereas the second row represents the ISL.