# Simplest formula of a calcium phosphate mineral?

Given the formula:

$$\ce{Ca}_x\ce{(PO4)}_y\ce{OH}$$

Find the lowest possible values of $x,y$ and state (look up) the mineral name.

Based on the anion charges I know, and trying to balance them, I would expect

$$\ce{2Ca^{2+} + (PO_{4})^{3-} + OH^{-} -> Ca_{2}(PO_{4})OH}$$

But I couldn't find a mineral with this formula, and the closest I could find was hydroxyapatite with the formula:

$$\ce{Ca_{5}(PO_{4})_{3}OH}$$

which is still balanced based on the charges I had initially used, but isn't the same. Can anyone explain why this mineral doesn't form as I had expected?

• Welcome to Earth Science Stack Exchange! Good question. I can't explain it, but I did find isoclasite which is a hydrous form of your expected mineral. – kwinkunks Sep 13 '15 at 20:05
• @kwinkunks Thanks so much! I didn't think to look for a hydrous form. I'm still sort of curious as to why it doesn't form anhydrous, but its good to know that my formula wasn't completely wrong. – Mecury-197 Sep 13 '15 at 22:22
• For a mineral to form you not only need the charges to balance but the ion sizes need to be able to fit together into a crystal structure. I suspect this is why you don't find your anhydrous mineral – haresfur Sep 15 '15 at 23:07