The crystal-structure and the high-pressure behavior of a natural cubic melanophlogite (Sp. Gr. Pm3n at room-conditions), a type-I SiO2 clathrate containing only methane as guest gas, was investigated by X-ray single-crystal diffraction at room-conditions and in situ high-pressure synchrotron powder diffraction with a diamond anvil cell and methanol:ethanol:water = 16:3:1 (MEW) mixture and silicon-oil (Si-oil) as pressure transmitting media up to about 6 GPa.

A cubic-to-tetragonal phase-transition was observed at P ⩾ 1.14 GPa, in both MEW and Si-oil runs. The tetragonal polymorph is stable up to the highest pressure in the run with MEW, but it switches back to cubic symmetry at P > 3.12 GPa in Si-oil. The different elastic behavior is ascribable to the non-hydrostatic conditions in Si-oil medium at P > 1 GPa.

The elastic behavior of melanophlogite was described fitting the P–V data with a Birch–Murnaghan Equation-of-State, showing similar bulk modulus values for the low-pressure cubic and high-pressure tetragonal phases [i.e. K0,L-cub = 23.4(2.4), K0,tetr = 22.2(9)].

The spontaneous strain for the cubic-to-tetragonal transition is almost completely governed by a compression along the c axis, with negligible contribution along a. An inverse behavior is found with respect to the tetragonal-to-cubic high-temperature phase-transition previously described for Mt. Hamilton melanophlogite. A Landau fit of the spontaneous strain (εss), related to the order parameter (Q) of the transition as εss ∝ Q, showed a first-order character of the P-induced phase-transition and a Pc = 1.2(3) GPa. Symmetry breaking spontaneous strain prevails respect to the volume strain, which is however present and significant.