Struvite crystals: how do they form?

Question

I'm in general chemistry, and I was wondering how ionic species crystallize and form the shapes they do, specifically in regard to the struvite precipitate $\ce{MgNH_4PO_4. 6H_2O}$.

Deconstructing the ionic species, we get $\ce{NH_4^+}$, which does not have a dipole moment (I think), $\ce{PO_4^{3-}}$, which also does not have a dipole moment, $\ce{H_2O}$, which has its dipole moment pointing towards the oxygen, and $\ce{Mg^{2+}}$, which is just an ion floating in solution.

Drawing the Lewis structures of these molecules and writing the Lewis structures, I'm not really sure how the crystal would form. But I do have a guess.

I feel as if the $+1$ formal charge on the nitrogen in $\ce{NH_4^+}$ will be attracted to one of the $-1$ formal charges on the oxygens in $\ce{PO_4^{3-}}$, while the magnesium ion will take in some of the free electrons on the other two oxygens. The $\ce{H_2O}$ will then form hydrogen bonds with the net dipole moment generated?

But this all sounds so hand-wavy to me, and I know it's probably wrong. Furthermore, I can't really visualize what this would look like. I honestly don't have a good understanding of how things work with crystallization. This is, after all, my first semester of chemistry.

If someone could give me tips and hints on how a struvite unit arranges its ions via electrostatic interactions to make something stable, I would appreciate it. Resources and pages in textbooks would be appreciated too!

Answer 1

As Ed V points out, the conditions of formation of struvite have many variables and the struvite crystals produced are variable. On the one hand, this could be a pleasant exercise of playing with atomic models, but on the other, it may be unproductive unless you were to use computer graphing to model various configurations.

Perhaps you could start with a simpler, but similar compound and develop a crystallization mechanism from that. Ammonium phosphate seems simpler, but isn't, because the third proton of H$_3$PO$_4$ is so weak that pH is still a significant variable, and you are also dealing with diammonium phosphate and double salts (Ref 1).

Another possibility is plain magnesium phosphate, but it appears in several forms and with several degrees of hydration, so it is not a simple crystal formation. (Ref 2) The anhydrous form is too unrelated to struvite. (Ref 3)

Stretching your imagination further, replacing the ammonium with potassium gives an interesting material that has applications in cementing. One article gives a simple picture of the "components" of KMgPO$_4$.6H$_2$O:

The octahedron of Mg(H$_2$O)$_6$$^+$$^2$, the tetrahedron of PO$_4$$^3$$^-$, and the sphere of K$^+$ are objects that could be played with to make a model crystal. The sphere could also suggest how the NH$_4$$^+$ ion (just assume it is almost spherical) fits into struvite. (Ref 4)

Struvite was the basis for a rapid repair patch for roadways some years ago (Ref 5), and has current applications in immobilizing radioactive waste. (Ref 6)

Ref 1. https://en.wikipedia.org/wiki/Ammonium_phosphate

Ref 2. https://en.wikipedia.org/wiki/Magnesium_phosphate

Ref 3. https://byjus.com/chemistry/magnesium-phosphate/

Ref 4. https://pubs.acs.org/doi/10.1021/acs.jpcc.7b01396#

Ref 5. US Patent 3,960,580 J.M. Gaidis et al.

Ref 6. https://www.sciencedirect.com/topics/engineering/magnesium-potassium-phosphate