The pressure-volume relations of a natural manganite [gamma-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) angstrom(3), isothermal bulk modulus K-0 = 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 K-ao = 93( 4) GPa, K-b0 = 62( 5) GPa, K-c0 = 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 P2(1)/c with B2(1)/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 ( beta-MnO2) and the hydroxides of InOOH-related structure which were isotopological to manganite (B2(1)/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.

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