This invention provides a method of preparing a ²-hematin crystal comprising a step of heating, the ²-hematin crystal obtained by such method, and a vaccine adjuvant composition containing the ²-hematin crystal. The ²-hematin crystal has a needle-like morphology, it has an average particle size of 0.6 to 1.2 µm, and it exhibits main peaks characteristics for angles of diffraction (2¸) of 7.4°, 12.2°, 21.6°, and 24.1° in an X-ray diffraction pattern obtained by powder X-ray diffractometry with Cu-K± rays (with each peak including a plus-minus 0.2° diffraction angle).
