A sample from the Koktokay No. 3 granitic pegmatite, Altai Mountains, northwestern China, which shows a transition from lithiophilite [LiMn2+PO4] to sicklerite [Li1–xMn2+PO4], was investigated by single-crystal X-ray diffraction, electron-microprobe analysis, and secondary-ion mass spectrometry (SIMS) techniques. Under the polarizing microscope, the sample shows colorless lithiophilite and deep-orange sicklerite, as well as several intermediate phases. The chemical compositions change from Li0.96(Mn2+0.81Fe2+0.09Fe3+0.08)PO4 to Li0.69(Mn2+0.62Mn3+0.19Fe3+0.16)PO4, and show a progressive decrease of the lithium content from lithiophilite to sicklerite. Five crystals were extracted from the thin section, and their crystal structures were refined in space group Pbnm, with unit-cell parameters from a 4.736(1), b 10.432(2), c 6.088(1) Å (lithiophilite), to a 4.765(1), b 10.338(1), c 6.060(1) Å (sicklerite). The olivine-type structures of these phosphates are identical to that of triphylite, and are characterized by two chains of edge-sharing octahedra parallel to the c axis. The first chain consists of M1 octahedra containing Li atoms and vacancies, and the second chain consists of M2 octahedra occupied by Fe and Mn. The significant Mn3+ content in sicklerite necessitates a more careful interpretation of the electron-microprobe data, and demonstrates the different mechanisms of oxidation affecting the lithiophilite–sicklerite and triphylite–ferrisicklerite series, respectively.