Phase Diagram of Halloysite under High Pressure and Temperature and Its Geophysical Implications

Hugoniot measurements for halloysite with two different initial densities have been performed at the shock pressures up to about 100 GPa. Three distinct regions appear along their Hugoniots. For the samples of 0=1.375 g/cm3, a low-pressure phase (LPP) exists within the shock pressure up to about 12.69 GPa, a mixed phase region (MP) begins at 12.69 GPa and ends at about 22.90 GPa, and then a high-pressure phase (HPP) occurs at shock pressures between 22.90 GPa and 46.64 GPa. The fitted linear D-u relations of its LPP and HPP can be expressed respectively as D=0.24+1.89u and D=2.47+1.12u, D is the shock wave velocity and u is the particle velocity (km/s). For the samples with 0=2.001 g/cm3, the pressure ranges of its LPP, MP are covering 0~35.77 GPa, 35.77~95.48 GPa, respectively, and no HPP obviously shows on its Hugoniot. The fitted linear D-u relations of its LPP and MP are D=1.73+1.72u and D=2.69+1.37u , respectively. p-T phase boundary is determined approximately by the Mie-Grneisen Equation of state using the present parameters. It is compared with the linear geothermal lines of 10 ℃/km and 4 ℃/km, and suggested that halloysite may be stable at depth about 50 km inupper mantle or as a transient phase in subducting slabs at depth of about 133 km.