Molecular site and infrared-absorption data suggest that the normally tetrahedral XY43− group in apatite- and pyromorphite-series minerals has C3v or lower symmetry. Basic differences in the absorption patterns of pyromorphite, mimetite, and vanadinite are related to differences in mass and binding energy of the functional ions, inversion of the frequency of the v3 and v1 vibrations and substitution of non-periodic molecular components such as V for P and As. Complete replacement of Ca by Pb in the apatite structure causes a shift of the v3 and v1 vibrations to lower frequencies, as expected.
From inspection of absorption spectra, phosphate groups substituting in mimetite and vanadinite can be distinguished from intergrowths of pyromorphite with mimetite or vanadinite even at low PO43− concentrations producing no detectable response in x-ray spectra obtained routinely. The PO43− molecule is apparently slightly expanded when substituting for AsO43− and VO43− ions.