Apatite is actually three different minerals depending on the predominance of either fluorine, chlorine or the hydroxyl group. These ions can freely substitute in the crystal lattice and all three are usually present in every specimen although some specimens have been close to 100% in one or the other. The rather non-inventive names of these minerals are fluorapatite, chlorapatite and hydroxylapatite. The three are usually considered together due to the difficulty in distinguishing them in hand samples using ordinary methods.