A sample from the Koktokay No 3 granitic pegmatite, Altai Mountains, northwestern China, which shows a transition from lithiophilite [LiMn PO ] to sicklerite [Li Mn PO ], 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 Li (Mn Fe Fe )PO to Li (Mn Mn Fe ) PO , 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 Mn 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.