The calculation you are doing is to account for (exclude) the presence of apatite (Ca5[PO4]3(OH, F, Cl), in which (ideally) there are 5 moles of CaO and 1.5 moles of P2O5. So, for every mole of P2O5 you need to subtract 3.33 (10/3) moles of CaO assuming it is all in apatite, then see how much CaO is left over.

However, depending on the rock type, not all the P2O5 (or indeed any of it) needs to be in apatite. For example, in metapelites, it might be in monazite. You should also check that you have converted your bulk compositions to molecular rather than weight percent values. Lastly, all analyses are associated with uncertainties that need to be accounted for.

What negative values are you getting?

Good luck.

There is nothing more to add. Tim has most fully answered your query. Wishing you well in your studies

George Strachan

Thanks, Tim! I read your answer carefully. That answers all my questions.