Ca 4 P2O9 Tetracalcium phosphate : C4P
HILGENSTOKITE is the mineral having the same chemical composition as Ca 4 P2O9 the most basic calcium orthophosphates
Chemical composition
The chemical composition of Ca 4 P2O9 is shown in table 1
Table 1
-

Mineralogical composition :
Ca 4 P2O9  cristallises in the monoclinic system. The crystallographic parameters are shown in table 2 and the XRD pattern is shown in figure 1

Table 2
-

Figure 1 XRD of  Ca 4 P2O9     (C4P)

-
An orthorhombic polymorph has been detected

Synthesis : Ca 4 P2O9 is obtained by firing an equimolar concentration of CaHPO4 and CaCO3 at 1500°C
The following steps have been analysed during the process of firing
a) decomposition of the precursor  CaHPO4-> Ca2 P2O7 +H2O
b) decarbonation CaCO3 ->  CaO+ CO2
c) a- Ca2 P2O +  CaO  -> Ca3 (P2O4)
d) Ca3 (P2O4) + CaO + H2O -> Ca10 (PO4) (OH)2    T<1200°C
e) Ca10 (PO4) (OH)2 + 2CaO-> Ca 4 P2O9    + H2O T°>1200°C

The thermal balance of the production Ca 4 P2O9 is given table 3
Table 3
Thermal balance (Kcal/kg)
-
Ca 4 P2O9 is stable up to 1610°C where it mets congruently
Hydration.of Ca 4 P2O9 Applications. Its solubility at 25 °C (0,0007g/l) is higher than  hydroxyapatite (HAP)(0,0003 g/l or fluoroapatite (0,0002 g/l) at the same temperature The hydration of Ca 4 P2O9 is of interest because it is the only calcium more basic than  HAP. and it is the base used in many aqueous process forming Ca10 (PO4) 6 (OH)2   (HAP) A complete hydration  would be expected to produce portlandite  Ca(OH)2 as follows:
3Ca 4 P2O9 + H2O ->  Ca10 (PO4) (OH)2  + Ca(OH)2
After a  short period of reaction, further hydration is very slow, A substantial heat is released during the period controlled interfacially  and HAP is precipitated.. Ca(OH)2 is precipitated during extended strorage. A continuous removal of the hydration product is  required to facilitate a high rate of reaction. An equimolar Ca 4 P2O9 and Ca HPO4 developed strengths by reaction with water et composed  calcium phosphate cements by a sketch of reactions as follows:
Ca H PO4-> Ca 2+ + H PO42-
and Ca 4 P2O9+ H2O ->  4Ca 2+ + 2  PO43- + 2 OH-