The pressure-volume relations of a natural manganite [γ-MnOOH] were
measured at ambient temperature and high pressures up to 12.7 GPa using a diamond anvil cell (DAC) equipment and synchrotron radiation X-ray diffraction technique. No phase transition has been observed within the whole pressure range in this study. Analysis of pressure-volume data on P21/c setting structure to a third-order Birch-Murnaghan equation of state yielded: zero-pressure volume V0 = 135.46(4) Å3, isothermal bulk modulus K0 = 86(2) GPa and its pressure derivative K’0 = 6.8(6). The axial
compression behavior shows that the manganite is a compression anisotropic mineral with Kao = 93(4) GPa, Kb0 = 62(5) GPa, Kc0 = 82(4) GPa,which reveals that the a-axis is the least compressible, then b-axis is the most compressible, and c-axis is between them. Comparing the compressibility of manganite based on P21/c with B21/d setting showed that the different of crystallization orientation was the unique reason for the different axial modulus. Furthermore, we also compared the compressional properties manganite with pyrolusite (β-MnO2) and the hydroxides of InOOH-related structure which were isotopological to manganite (B21/d),and inferred that the combinations of Jahn-Teller effect and hydroxyl were the major reasons for the far less bulk modulus of manganite than that of the pyrolusite, and the Jahn-Teller effect and the least electronegativity central coordinated cation-Mn3+ in manganite were the reasons for having the smallest bulk modulus of manganite among the InOOH-related hydroxides.