Formation of Biphasic Hydroxylapatite-Beta Magnesium Tricalcium Phosphate in Heat Treated Salmonid Vertebrae

Abstract

Ichthyoarchaeological evidence is uncommon at ancient hunter-gatherer sites from various regions and timeframes. This research contributes to the development of microarchaeological techniques useful for identifying fishing economies in situations where classifiable bones are unavailable. Specifically, traces of heat altered bone mineral in domestic hearths are expected to provide markers for discarded fish remains. We used a series of laboratory incineration experiments to characterize the mineralogy of burned salmonid vertebrae. Fourier transform infrared spectroscopy and x-ray diffraction distinguished the formation of beta magnesium tricalcium phosphate (βMgTCP) at temperatures as low as 600 °C. Bones from a sample of game mammals and birds did not form this phase at temperatures below 1,000 °C. We propose that this neoformed mineral can serve as a proxy for hunter-gatherer salmonid fishing when typical ichthyoarchaeological evidence is absent. Using Fourier transform infrared spectroscopy, it will be possible to rapidly and inexpensively determine the presence of βMgTCP in fragmentary burned bone remains associated with combustion features. The occurrence of βMgTCP in archaeological hearth features will offer a new means of further evaluating the temporal, geographic, and cultural scope of salmonid harvesting. We also acknowledge the value of biphasic hydroxylapatite-βMgTCP recovered from Atlantic salmon vertebrae as a bioceramic.

Figure 1

Content of Acid Insoluble Organic Matter Extracted from Unheated Air-Dried Salmonid, Codfish, Caribou, Moose, and Duck Bones. Results illustrate greater organic matter contents in salmonid specimens.

Figure 2

Mass Losses in Salmonid, Codfish, Caribou, Moose, Duck, and Ptarmigan Bones throughout the Heat Treatment Process. All species lost a similar amount of weight at 200 °C owing to evaporating H₂O and melting fats; salmonid specimens show a sharp decrease at 400 °C resulting from greater losses of their larger organic matter fractions; caribou, moose, and duck contain the lowest amounts of organic matter and show similar mass loss trajectories.

Figure 3

FTIR Spectra of Salmon and Codfish Bones Heat Treated through a Ramping Temperature Sequence between 200 °C and 1,000 °C. (A) Atlantic Salmon; note the loss of the HPO₄/CO₃ peak and appearance of strong βMgTCP peaks at 600 °C; (B) Atlantic Cod; the loss of the HPO₄ / CO₃ peak and appearance of weak βMgTCP peaks occurs at 800 °C; abbreviations: Tgy = triglyceride ester; Am = amide; Alk = alkene; Pr = proline; CO₃ = carbonate; PO₄ = phosphate; HPO₄ = hydrogen phosphate; Arom = aromatic char compounds; WH = whitlockite; β = beta magnesium tricalcium phosphate.

Figure 4

FTIR Spectra of Caribou and Duck Bones Heat Treated through a Ramping Temperature Sequence between 200 °C and 1,000 °C. (A) Caribou; βMgTCP does not form; (B) Duck; βMgTCP is only formed at high temperatures; abbreviations: Tgy = triglyceride ester; Am = amide; Alk = alkene; Pr = proline; CO₃ = carbonate; PO₄ = phosphate; HPO₄ = hydrogen phosphate; Arom = aromatic char compounds; WH = whitlockite; β = beta magnesium tricalcium phosphate.

Figure 5

Magnified Representations of the Phosphate v3 and v4 FTIR Bands Demonstrating the Formation of Whitlockite and Beta Magnesium Tricalcium Phosphate Phases in Heat Treated Atlantic Salmon Vertebrae. (A) PO₄ v3 band of a salmon caudal vertebra treated at 500 °C; diagnostic WH shoulders appear at 1,150 cm^–1 and 990 cm^–1; the presence of HPO₄ demonstrated by the 880 cm^–1 peak distinguishes WH from βMgTCP; (B) changes in the PO₄ v4 band of a salmon caudal vertebra treated at 500 °C; (C) PO₄ v3 band of a salmon caudal vertebra treated at 600 °C; loss of the diagnostic 1,150 cm^–1 WH peak and the 880 cm^–1 HPO₄ peak and appearances of the 984 cm^–1 and 945 cm^–1 peaks signify a shift from WH to βMgTCP at 600 °C; (D) changes in the PO₄ v4 band of a salmon caudal vertebra treated at 600 °C; abbreviations: HAp = hydroxylapatite; WH = whitlockite; β = beta magnesium tricalcium phosphate; HPO₄ = hydrogen phosphate.

Figure 6

Observed XRD Diffractograms and Rietveld Refinements for Heat Treated Atlantic Salmon Bone. Each inset displays the observed diffractogram on top followed by refinement results on bottom; diagnostic reflectance planes and corresponding 2 θ angles for each phase are indicated in the observed diffractograms; separated phases are presented in the refinement sections; hydroxylapatite (HAp) is represented in black, whitlockite (WH) in blue, and beta magnesium tricalcium phosphate (βMgTCP) in red; (A) salmon caudal vertebra heat treated at 500 °C demonstrating diagnostic reflectance peaks for WH and HAp; (B) salmon caudal vertebra heat treated at 600 °C demonstrating diagnostic reflectance peaks for βMgTCP and HAp; (C) well developed biphasic HAp-βMgTCP in a salmon caudal vertebra after treatment at 800 °C.

Figure 7

Mineral Phase Percentages in Heat Treated Atlantic Salmon Bone in Relation to the Ca/P Ratio of the Biphasic Mineral. Results demonstrate an inverse relationship between βMgTCP formation and the Ca/P ratio of the Biphasic Mineral.