The high-pressure behaviour of the “moganite” polymorph of SiO2 has been investigated by angle-dispersive, X-ray powder diffraction in a diamond anvil cell at room temperature. The bulk modulus was calculated from the P-V data: B0 = 32.2(3) GPa with B’0 fixed to 5.2 as for quartz. The molar volume of moganite becomes lower than the volume of quartz above 5 GPa. Pressure-induced amorphisation occurred progressively; it was complete above 25 GPa. Upon decompression, amorphisation was partially reversible if the maximum pressure reached was below 25–30 GPa and irreversible when the pressure reached was above 45 GPa. The pressure-induced amorphisation of the quartz and moganite polymorphs of silica and phosphorus oxynitride PON, a silica analogue, is related to the distortion of the tetrahedra and, in the case of silica, to the presence of a much denser phase, in which the cation has a higher coordination number.