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I had a homework question:

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?

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  • $\begingroup$ 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. $\endgroup$
    – Matt Hall
    Sep 13, 2015 at 20:05
  • $\begingroup$ @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. $\endgroup$
    – Mecury-197
    Sep 13, 2015 at 22:22
  • $\begingroup$ 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 $\endgroup$
    – haresfur
    Sep 15, 2015 at 23:07

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Yes, you are correct in your calculation. However, the most common hydrated calcium phosphate in nature is indeed hydroxyapatite. Note that industrially, there are several other calcium phosphates (see link, note that none of them is your calcium phosphate) The reason that it forms instead of the "simpler" mineral is that it is more stable thermodynamically. I'm not certain about the exact reasons for that.

Apatite is so stable that in nature and rocks, almost whenever you have phosphate, it binds with calcium to form apatite. The only way to not have apatite is to not have calcium, not have phosphate, or have some really exotic system that suppresses its formation.

This is also known experimentally. In experiments that simulated calcium phosphate systems as early as the 60s, it was hard to crystallise anything but apatite.

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