FTIR spectra (cm⁻¹) peak position of stoichiometric and nonstoichiometric phases in the brachiopod shells. The peaks overlap with the (type I) collagen and chondroitin sulfate A (glycosaminoglycan; GAG) peaks^a.

Type I collagen	Chondroitin sulfate A	D. tenuis	L. anatina (ML/RL)	Assignment
	1122	1116	1126/21	HPO42− stoichiometric apatite overlap with GAGs
1096	1094	1095	1092/1101	CO32− and/or HPO42−groups
1081		1080	1080/1	vC–O in collagen and GAGs overlaps with v3PO43−
1063	1062	1067	1060/3	vC–O in collagen and GAGs overlaps with lipids and v3PO43−
1046		1053	1043	vC–O carbohydrate residues in collagen and GAGs/v3PO43−
		1037	1038	PO43−groups in OCP
1031	1027	1027	1030/2	vC–O carbohydrate residues in collagen and GAGs overlap with vasPO43− group of stoichiometric apatite
			1025	v asPO43−
1019	1018	1022	1019/21	v asPO43− and/or CO32− group in nonstoichiometric apatite
	992	997		v asPO43− in apatite environment overlap with GAGs
971			973	v 1PO43−
		967	965/8	v 1PO43−

^aNote: peaks were derived from second derivative of deconvoluted spectra and their assignments from literature values.^{26,29,34,44,50,52}