Table 1. List of CaP phases.

Category	Name	Symbol	Formula	Ca/P	Mineral
Low-temperature CaPs	Monocalcium phosphate monohydrate	MCPM	Ca(H2PO4)2·H2O	0.50	-
	Dicalcium phosphate	DCP	CaHPO4	1.00	Monetite
	Dicalcium phosphate dihydrate	DCPD	CaHPO4·2H2O	1.00	Brushite
	Octocalcium phosphate	OCP	Ca8H2(PO4)6·5H2O	1.33	-
	Precipitated hydroxyapatite	PHA	Ca10-x(HPO4)x(PO4)6-x(OH)2-x	1.50–1.67	-
	x = 1	CDHA	Ca9(HPO4)1(PO4)5(OH)	1.50
	x = 0	HA	Ca10(PO4)6(OH)2	1.67
	Amorphous calcium phosphate	ACP	Ca3(PO4)2·nH2O where n = 3–4.5; 15–20% H2O	1.50	-
High-temperature CaPs	Monocalcium phosphate	MCP	Ca(H2PO4)2*	0.50	-
	α-Tricalcium phosphate	α-TCP	α-Ca3(PO4)2	1.50	-
	β-Tricalcium phosphate**	β-TCP	β-Ca3(PO4)2	1.50	-
	Hydroxyapatite	HA	Ca10(PO4)6(OH)2	1.67	Hydroxyapatite
	Oxyapatite***	OXA	Ca10(PO4)6O	1.67	-
	Tetracalcium phosphate	TetCP	Ca4(PO4)2O	2.00	Hilgenstockite

The first phases can be obtained at or close to room temperature: they are called “Low-temperature CaPs.” The last 6 phases can only be obtained at temperatures above 100°C and hence are called “High-temperature CaPs.” Thermodynamically, hydroxyapatite (HA) is the most stable phase above a pH value close to 4.5¹⁵¹ but only readily precipitate above pH 7.0–7.5. Interestingly, the Ca/P molar ratio of precipitated HA (PHA) tends to vary according to the synthesis conditions, being lower in neutral pH conditions than in basic pH conditions. When the Ca/P molar ratio is equal to 1.50, one refers to “calcium-deficient hydroxyapatite” (CDHA). The typical size of PHA crystals is below 100nm. Since HA is stable at high temperature, HA can also be formed by solid state reaction. Even though the composition is the same as that of PHA, the crystal size is much bigger. *Could be also classified under “low-temperature CaPs” because MCP can be obtained by dehydration of MCPM just above 100°C¹⁵²; **Can also be obtained by precipitation in organic media^31,56; ***Very difficult to synthesize because it is extremely hygroscopic.¹⁵³