SOLID SOLUBILITY OF APATITES IN SILICOPHOSPHATES AND SILICOSULPHATES

Two distinct phases containing phosphorus, silicon and sulphur occur in cement clinker and rhinania fertilizer.-These are solid solutions based on two structural types: apatite (phosphate apatites and ellestedites) and silicocamotite (silicocarnotite and silicosulphate) structures. Ellestedites, Calo(Si04)3(S04)3 and apatites, Calo(P04)6 22 (Z = F, C1 or OH) are. completely miscible as are silicocarnotite, Calo(Si04)2(P04)4, and silicosulphate, Calo(Si04)4(S04)z. The silicocarnotite, however, shows limited solid solubility with apatites. Ternary phase diagrams constructed for the systems fluorapatite chlorapatite silicocamotite, fluorapatite chlorapatite silicosulphate, fluorellestedite chlorellestedite silicocarnotite and fluorellestedite chlorellestedite silicosulphate show that they all have three distinct phase regions. These are apatite solid solution, silicocamotite solid solution and a two phase region containing the two solid solutions. The extent of solubility is different in the four systems. A noteworthy feature is that ellestedites 0 form extensive halogen-deficient solid solutions with silicosulphate at 900-1100 C. In all cases the solid solution may be'envisaged by substitution of P+' by equal number of ~ i + ~ and s + ~ in tetrahedral positions (2~+'~ i + ~ + ~ + q . The results are important for understanding the thermal behaviour of industrially important materials containing P, Si and S.


INTRODUCTION
Apatites are chemically and industrially important materials. Apatite structure1 is fairly stable under varying conditions and is adopted by many chemically similar phases. Related silicon and sulphur containing phases such as silicosulphate and silicocarnotite have been widely reported2 to occur in cement clinker and in rhenania fertilizer3 made from apatite.
TWO structural types may be recognized among these compounds with apatite and silicocarnotite structures. Crystal data for these phases are summarized in Table 1. Natural and synthetic apatites containing P, Si and S have the general formula Calo(X04)6Z$ typically, X = P or (Si + S) and Z = F, C1 or OH. Apatites, Silicocarnotite, CaS(Si04)(P04)2 and silicosulphate, Cas(Si04)2 SOs are believed to be isostructural.
It is generally assumed that all apatite members are completely miscible and that the silicocarnotite members are also completely miscible. However there has been no systematic study of the solid solubility of apatites in silicocarnotite type phases. The extent of solid solubility of haloapatites in silicocarnotite and silicosulphate in the temperature range 900 -1 1 0 0~~ was determined in the present investigation. The results are important for understanding the thermal behaviour of industrially important materials containing tetrahedral phosphorus, silicon and sulphur.
The products were then examined by powder X-ray diffraction using a Hagg Guinier camera with Cu K, radiation. In most cases pure samples of the end members were synthesized and their X-ray powder patterns were taken as standard patterns after verification. X-ray powder patterns of the heat-treated products were compared with the standard powder patterns on an illuminated screen for rapid identilication. Representative samples were also examined under a polarizing microscope to conlirm the absence of any melts or glass after firing.

RESULTS
Preliminary experiments disclosed that solid solution between fluorapatitechlorapatite, haloapatitehydroxyapatite and fluorellesteditechlorellestedite are essentially complete. Variation of unit cell dimension, 5 with substitution of F or OH by C1 in apatites has also been studied in the solid solution series. The results are shown in Figure 1. Continuous increase of g axis is observed with substitution of FIOH by C1.

Figure 1: Variation of unit cell dimension (in A@) with subslitutlon of FIOII by CI In apatites
The silicocarnotite join was also found to be binary with complete solid solubility across the join. Wlth this information heating experiments were conducted on the following ternary systems at 1100~~.
The results of these heating experiments are summarized in Table 2. On the basis of these results phase diagrams were constructed for the above mentioned 4 ternary systems at 1 1 0 0~~. Figure 2 shows the phase diagram constructed for the system at 1 1 0 0~~. A ' complete series of solid solution was observed on the FAp-CL4p binary join while limited solid solubility was observed along the other two binary joins with silicocarnotite. The plane shown (Fig. 2) is a true ternary system at subsolidus temperatures and contains 3 distinct regions. I k o of these are small fields: one of homogeneous silicocarnotite, the other of apatite solid solution. It also contains a large phase region in which the two solid solutions coexist.

Fluorellestedite-chlorellestedite-silicocarnotite system
Fluorellestedite and chlorellestedite were found to be completely miscible while the joins involving silicocarnotite had partial solid solubility at both ends. The ternary phase equilibrium diagram (Fig. 4) shows three distinct regions: a very narrow region of silicocarnotite s.s. and a large region of ellestedite S.S. and a much larger two-phase region in between. As shown in the diagram the extent of solid solubility along CIE1ls:Sc is more extensive than in the FElls-Sc join.
Fluorellestedite-chlorellestedite-silicosulphate system This system includes the compositions of the two halogen Si -S apatites and silicosulphate. Figure 5 shows the results of this study. Although silicosulphate and ellestedite are not completely miscible, appreciable solid solubility exists at both ends of this binary join. This plane is also a true ternary system at subsolidus temperatures.
It also has 3 distinct regions as in the previous cases . The most significant difference is the presence of an extensive' field of single-phase ellestedite (apatite type) solid solutions. These extend to 50-60% mol silicosulphate, which is the most extensive apatite based ternary solid solution observed in the present study.   Some selected compositions representing each of the systems meniioned above were annealed again at 9 0 0 '~ for 6 days to see whether exsolution occurred. No changes were observed in the X-ray powder patterns after this heat treatment indicating that the limits of solid solution remained essentially unaltered event at 900'~. Therefore, it seems likely that the observed ranges of apatite solid solution are a feature of these phase diagrams over a wide range of temperatures.

DISCUSSION
The study demonstrates that there is a complete series of solid solution between fluorapatite and hydroxyapatite as well as between fluorapatite and chlorapatite. Although it appears that the three phases are isostructural, in fact only fluorapatite and hydroxyapatite are strictly i s o s t r~c t u r a l .~~ Chlorapatite structure is different from the other two with regard to the halogen positions.10 In the apatite structure the vertical chains of calcium and oxygen atoms are linked by X04 groups to form honeycomb structure with channels parallel to the c: axis. Halide ions are positioned within these channels. In hydroxy-and fluorapatites the OH and F are situated on mirror planes at (00114) and (003/4), whereas in chlorapatite the Cl atoms are at (000) and (00112) positions which are relatively large sites. Since the distances between two sets of sites are not large enough to accommodate halogens at both sites, the occurrence of complete series of solid solutions between fluor-and chlorapatites cannot be explained using a direct replacement mechanism.
Crystal structure investigation of a natural fluorchlorapatitell has revealed that the solid solution in fluorchlorapatite is achieved by a 0 . 4~' shift of the C1 atoms along c axis, relative to its position in end-member chlorapatite. It has also been suggested from X-ray data12 that fluorchlorapatite is made possible by creation of new F sites in the anion column, which yields reasonable CI-F distances. Therefore, it appears that the solid solution mechanism in this case is rather complex and involves either shifting of existing CI sites or creation of new FIOH sites to reduce any strain in the structure.
Chlorellestedite and fluorellestedite are isostructural and they too have the apatite . structure. Silicocarnotite and. silicophosphate, on the other hand, have a. somewhat different structure related to apatite framework. As such, complete solubility has been observed in chlorellesteditefluorellestedite, apatiteellestedite and silicocarnotitesilicosulphate binary systems. It is also worth noting that more extended solid solubility has been observed (Figs. 2-4) between ellestedite-silicosulphate than in the case of apatite-silicocarnotite, owing to their closer chemical and structural resemblance in the former case.
Apatite-ellestedite solid solution series is formed by the replacement of P" in ~0 7~ groups by equal nurnblr of Si+4 and S+6 as shown below: Similarly, silicocarno'tite-silicosulphate solid solution series may be visualized as follows: Solid solutions between apatite and a halogen-free phase such as silicosulphate implies the existence of apatites deficient in halogen relative to the ideal formula. Thus, it can be seen that the solid solutions extending from ellestedite towards silicosulphate must therefore have the general formula,

(0 represent halogen vacancies)
This solid solution series containing vacant halogen sites is rather extensive with x reaching up to about 1.3 (Fig. 5). This is not unusual : halogen deficient apatites in which halogens are replaced by 02' have been reported.13 Recent structure determination of an o~~a~a t i t e~~~~ has confirmed its widespread occurrence. However, in this solid solution series no extra oxygens are needed to achieve a charge balance. Therefore, this solid solution series represents a novel group of halogen deficient calcium apatites.