Phosphates with Ion Vacancies

In general, the substitution of 804 for PO in an orthophosphate (or substitution of PO for SiO in a silicate) will result in fewer cations being required for charge balance. This may lead to a structure containing vacancies unless some cations are replaced with cations of smaller charge as in 

On the other hand, substitution of SiO for PO in an existing phosphate structure (or substitution of P04 in a sulphate) requires either extra interstitial cations of the same kind as in Ca,o+ c(P04) 2x(Si04)2j Fj or replacement cations with higher charge. 

Some varieties of monazite [9], CeP04, contain impurity Th +, and replacement of nCe by 3/4n Th + maintains charge balance and creates unoccupied cation sites. On the other hand, vacancies are not produced if simultaneous substitution of Th 4- SiO for Ce + 4- PO takes place, and if this process is carried to completion the isomorphous huttonite, ThSi04, is obtained, as already mentioned in Section 5.3. 

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