I wasn't going to reply but there are some issues here - namely the composition of the apatite.

What other oxygen sources are in the apatite?

Many apatites such as dahlite and francolite contain carbonate as part of the structure.  You must separate the carbonate oyxgen first as the phosphate and carbonate have different equilibrium functions.

Other apatites (dahlite, hydroxylapatite) contain OH-.  This also has to be removed by chemical processing first.

Bone apatite in general is very complex and not really a pure chemical.  Another problem you may encounter with it is the organic material intimately associated with it.

So, yes, you can analyse a pure fluorapatite by fluorination without processing but that is really an exception.

With regard to monazite, I seem to remember that the structure is hexagonal like bismuth phosphate.  BiPO4 (as older workers in this field will remember) contains  molecule of water per BiPO4 in the form that it initially crystallizes.  Does monazite also contain structural water?

Good luck