A sample of bertossaite {CaLi2[Al4(PO4)4(OH,F)4]} from the Buranga pegmatite, Rwanda, was investigated by single-crystal X-ray diffraction, electron microprobe, and infrared spectroscopic techniques. Under the polarizing microscope, bertossaite forms large xenomorphic grains reaching 500 μm in length, included in montebrasite and associated with quartz and a phosphate of the lazulite–scorzalite series. Electron-microprobe analyses indicate the empirical formula (Ca0.83Na0.15Sr0.02)∑1.00(Li1.86Mn0.10 Na0.04)∑2.00[(Al3.90Fe0.09Mg0.01)∑4.00(PO4)4(OH3.35F0.65)∑4.00]; the mechanism of coupled substitution Li+ + Ca2+ = Mn2+ + Na+ explains the presence of Na at the Ca site and of Mn at the Li site. The crystal structure of bertossaite [Imcb, a 11.476(1), b 15.744(1), c 7.228(1) Å] is isotypic with that of palermoite {SrLi2[Al4(PO4)4(OH)4]}, and is based on dimers of edge-sharing AlO6 octahedra, which are connected together via corner-sharing to form chains of octahedra running along the a axis. These chains are connected to adjacent chains by corner-sharing PO4 tetrahedra, constituting a heteropolyhedral framework that contains large channels in which the Ca and Li atoms occur. A comparison between the OH…O distances calculated from the infrared spectrum, and those obtained from the structural data, allows one to assign the bands corresponding to the stretching vibrations of the (OH) groups.

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