| Summary: | This study investigates the crystal chemistry of monazite (<i>A</i>PO<sub>4</sub>, where <i>A</i> = Lanthanides = Ln, as well as Y, Th, U, Ca, and Pb) based on four samples from different localities using single-crystal X-ray diffraction and electron-probe microanalysis. The crystal structure of all four samples are well refined, as indicated by their refinement statistics. Relatively large unit-cell parameters (<i>a</i> = 6.7640(5), <i>b</i> = 6.9850(4), <i>c</i> = 6.4500(3) Å, <i>β</i> = 103.584(2)°, and <i>V</i> = 296.22(3) Å<sup>3</sup>) are obtained for a detrital monazite-Ce from Cox’s Bazar, Bangladesh. Sm-rich monazite from Gunnison County, Colorado, USA, has smaller unit-cell parameters (<i>a</i> = 6.7010(4), <i>b</i> = 6.9080(4), <i>c</i> = 6.4300(4) Å, <i>β</i> = 103.817(3)°, and <i>V</i> = 289.04(3) Å<sup>3</sup>). The <i>a</i>, <i>b</i>, and <i>c</i> unit-cell parameters vary linearly with the unit-cell volume, <i>V</i>. The change in the <i>a</i> parameter is large (0.2 Å) and is related to the type of cations occupying the <i>A</i> site. The average <<i>A</i>-O> distances vary linearly with <i>V</i>, whereas the average <P-O> distances are nearly constant because the PO<sub>4</sub> group is a rigid tetrahedron.
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