# What does the unit cell of petitjeanite look like?

The question in Chemistry SE What is the crystal structure of bismuth oxyhydroxyphosphate (BOHP)? is about a catalyst synthesized by accident. A link in this comment points to the page https://www.mindat.org/min-3329.html which is for the mineral petitjeanite, or $$\ce{Bi3(PO4)2O(OH)}$$. There are about 38 photos available there, and there seems to be quite a diverse range of appearances to my untrained eye.

I was wondering if a drawing or computer-generated image of a unit cell for petitjeanite would exist in any database or website for mineralogy and crystal structures, something like those shown in these questions 1, 2, 3.

Should one assume that the mineral petitjeanite and the chemical discussed in the recent Chemical & Engineering News article Photocatalyst shreds drinking water contaminant PFOA are probably the same crystal configuration and unit cell, or can there be some variety?

The paper also includes this SEM image of microcrystals of the "inadvertently produced bismuth oxyhydroxyphosphate (BOHP)."

The accidentally synthesized BOHP (shown) can break down the toxic industrial contaminant PFOA faster than any other photocatalyst. Credit: Environ. Sci. Technol. Lett.

Should one assume that the mineral petitjeanite and the chemical discussed in the recent Chemical & Engineering News article Photocatalyst shreds drinking water contaminant PFOA are probably the same crystal configuration and unit cell, or can there be some variety?

No you cannot assume the structure from the formula, many materials have polymorphs (think graphite vs. diamond). To know if the structures are the same you must look at the diffraction pattern from the paper. In the paper the synthesized bismuth oxyhydroxyphosphate matches that of petitjeanite so in this case that is the prototype structure.

Using the information about diffraction, from the ICDD Databse (pdf 01-082-8058)we find that petitjeanite is a triclinic system of the $$\mathrm P\bar 1$$ space group (#2). with lattice parameters of:

$$\require{mediawiki-texvc}$$ $$a = 9.798\AA; ~ b = 7.250\AA; ~ c= 6.866 \AA; \qquad \alpha = 88.280^\circ \beta = 115.270^\circ ~ \gamma = 110.700^\circ$$

and atomic positions of: $$\begin{array}{c|lll} \text{Atom}&x&y&z\\ \hline \ce{Bi}&~~~0.52327&~~~0.41421&~~~0.7523\\ \ce{Bi}&~~~0.29838&~~~0.74044&~~~0.55549\\ \ce{Bi}&~~~0.07735&~~~0.1609&~~~0.28045\\ \ce{P}&~~~0.6952&~~~0.0172&~~~0.9215\\ \ce{P}&~~~0.0961&~~~0.372&~~~0.7862\\ \ce{O}&~~~0.3777&~~~0.5382&~~~0.4759\\ \ce{O}&~~~0.3066&~~~0.1549&~~~0.5579\\ \ce{O}&~~~0.859&~~~0.0275&~~~0.8943\\ \ce{O}&~~~0.5569&-0.2084&~~~0.8602\\ \ce{O}&~~~0.621&~~~0.1596&~~~0.7526\\ \ce{O}&~~~0.7684&~~~0.1052&~~~1.1811\\ \ce{O}&~~~0.2584&~~~0.5101&~~~0.7497\\ \ce{O}&-0.0468&~~~0.4637&~~~0.6902\\ \ce{O}&~~~0.0209&~~~0.1498&~~~0.6362\\ \ce{O}&~~~0.1695&~~~0.3727&~~~1.0506\\ \end{array}$$

and last but not least this renders to an image of:

Purple: Bi
Orange: P
Red: O

Having the same formula does not mean that the crystal structure is the same. An example is calcium carbonate, which can form the minerals calcite or aragonite. So you have to read the publications to know if they have done X-ray diffraction to positively identify the crystals as petitjeanite. If that is the case, then the crystal structure is triclinic and the relevant data can be found at webmineral.com.

ETA: Also note that the unit cell does not necessarily correspond to the visible crystal habit.