Revisiting the Phase-Equilibrium Diagram of the Ca 3 (PO 4 ) 2 –CaMg(SiO 3 ) 2 System Rau´ l Garcı´a Carrodeguas, w,z,z Antonio H. De Aza, z Ismael Garcı´a-Pa´ ez, z,y Salvador De Aza,** ,z and Pilar Pena z z Instituto de Cera´ mica y Vidrio, CSIC, 28049 Cantoblanco, Madrid, Spain y Departamento de Disen˜ o Meca´ nico, Materiales y Procesos, Facultad de Ingenierı´a, Universidad Francisco de Paula Santander, Colsag, San Jose´ de Cu´ cuta, Colombia z Centro de Biomateriales, Universidad de la Habana, La Habana 10400, Cuba As Si and Mg stimulate osteogenesis and mineralizing mecha- nisms, compositions belonging to the system Ca 3 (PO 4 ) 2 –CaMg (SiO 3 ) 2 , able to release these elements in situ, are promising can- didates for preparing ceramic bone implants. An exact knowledge of the phase relationships in the system is required to design ma- terials with controlled phase composition and properties. Thus, ceramic cylinders containing Ca 3 (PO 4 ) 2 (40, 45, 80, 90, 95, 99, and 100 wt%) and CaMg(SiO 3 ) 2 (balance to 100 wt%) were prepared and equilibrated at temperatures ranging from 11001 to 15001C. Phase composition of the resulting ceramics was studied by X-ray diffraction, thermal analysis, reflected-light optical microscopy, and scanning electron microscopy with attached energy-dispersive spectroscopy. Two invariant points were established. An eutectic point, previously reported, was confirmed at 13001751C and 37 wt% of Ca 3 (PO 4 ) 2 , where CaMg(SiO 3 ) 2 and b-Ca 3 (PO 4 ) 2 solid solutions coexist. A new peritectic point, corresponding to the reaction b-Ca 3 (PO 4 ) 2 solid solution2a-Ca 3 (PO 4 ) 2 solid solution1liquid was found at 144017101C and  70 wt% of Ca 3 (PO 4 ) 2 . Two new biphasic domains, where b- and a-, and a- and a 0 -Ca 3 (PO 4 ) 2 solid solu- tions coexist, respectively, are also proposed. The results showed that, at the eutectic temperature, the b-Ca 3 (PO 4 ) 2 solid solution extends up to  17 wt% CaMg(SiO 3 ) 2 . It was also found that the Si/Mg molar ratio in Ca 3 (PO 4 ) 2 solid solutions decreases with regard to pure CaMg(SiO 3 ) 2 , which might be explained through the incongruence of the dissolution process. I. Introduction C alcium phosphate-based bioceramics have been success- fully used in traumatology, dentistry, and maxilla–facial surgery for over 30 years for bone repairing, primarily because of their biocompatibility, bioactivity, and osteconductivity. 1 Commercial calcium phosphate bioceramics are usually com- posed of hydroxyapatite (Ca 10x (HPO 4 ) x (PO 4 ) 6x (OH) 2x , with 0rxr1), b-tricalcium phosphate (b-Ca 3 (PO 4 ) 2 ) or their admixture, hereafter named HAp, b-C 3 P, and biphasic calcium phosphate (BCP), respectively. 2–4 In spite of their acceptance, the bioresorption rate of HAp bioceramics is very slow and they remain undegraded several years after implantation. 4 BCP and b-C 3 P bioceramics are more reabsorbable than HAp ones; 2,3,5 however, even higher reabsorbtion rate is desirable for bone repairing applications when complete osseointegration and re- placement of the implant in a moderate term is required. In the last decade, intensive research has been devoted to prepare ion- substituted calcium phosphate materials to improve osteogen- esis, bioreabsorption rate, and strength. Particularly, Si 6–15 and Mg 16–22 have received great attention as substituents in calcium phosphates for biomedical applications, specifically in HAp and C 3 P. Silicon plays an essential role in the metabolic events that conduce to endochondral and intramembranous bone forma- tion, 23,24 and together with Ca, Na, and P (as released from 45S5 Bioglass s ), act on the expression of certain genes respon- sible for controlling the cell cycle of animal and human osteo- blasts and stimulates osteoproduction. 25–27 On the other hand, it has been proven that severe and also moderate dietary re- striction of Mg results in intensely depressed rates of bone for- mation and bone reabsorption, and affects normal bone development and bone strength in young growing rats. 28 Al- ternatively, injectable formulations of Mg-containing calcium phosphates are effective in improving the bone properties of ovariectomized rats and may be useful in osteoporosis ther- apy. 17 Also, clinical trials have shown that adequate Mg intake slows down menopausal osteoporosis and prevents the onset of senile osteoporosis in humans. 29 According to the above considerations, calcium phosphates with Si and Mg substitutions should be promising candidates for preparing bioceramics with improved osteogenic proper- ties. They should be biodegradable in order to provide the adequate levels of Si, Ca, P, and Mg ions to the biological environment as they are replaced by new bone tissue. Many attempts to introduce Si and Mg into the HAp network as a way to improve the bioactivity level of the material has been carried out. 6,7,9–13,16,20,30–44 However, HAp is the less soluble and reactive, and the denser and more packed of all calcium phosphates of biological relevance. On the other hand, C 3 P is more soluble and biodegradable than HAp, and is expected to contain that ions substituted in the C 3 P network that are more labile than in the HAp network as well. There are three polymorphs of C 3 P: 45 the low temperature, b-C 3 P, and the two high temperature forms, a- and a 0 -C 3 P. The last one lacks of practical interest because it only exists at temperatures aboveB1430 1C and reverts to a-C 3 P by cooling below the transition temperature. However, b-C 3 P is stable at room tem- perature and transforms reconstructively 46,47 atB1125 1C to a-C 3 P, which can be retained during cooling to room temper- ature. 48 Several commercial mono or biphasic bioceramics and composites are composed of b-C 3 P and a-C 3 P is the major constituent in powders of various hydraulic bone cements. 49 Dissolution and biodegradation rates are in the order a- C 3 P>b-C 3 P>HAp, 5 and they can be strongly influenced P. W. Brown—contributing editor This work was supported by Science and Technology Inter-Ministry Commission of Spain under project CICYT MAT2006-12749-C02-01 and by the State Secretary of Uni- versities and Research of the Ministry of Education and Science of Spain under Grant SAB2005-0015. w Author to whom correspondence should be addressed. e-mail: rgc@biomat.uh.cu **Fellow, The American Ceramic Society. Manuscript No. 26401. Received June 26, 2009; approved September 2, 2009. J ournal J. Am. Ceram. Soc., 93 [2] 561–569 (2010) DOI: 10.1111/j.1551-2916.2009.03425.x r 2009 The American Ceramic Society 561