In this paper, high-pressure data from a synchrotron X-ray diffraction study on a lillianite (Pb3Bi2S6) single crystal up to similar to 21 GPa are presented. A phase transition from lillianite (space group Bbmm, LP lillianite) to the high-pressure form beta-Pb3Bi2S6 (space group Pbnm, HP lillianite) was confirmed and bracketed between 4.90 and 4.92 GPa. The transition is reversible but of first-order with a hysteresis of similar to 2.8 GPa. It showed weak effects of pseudo-merohedral twinning that disappeared upon decompression, testifying to a full recovery of the single crystal of lillianite This makes lillianite an interesting shape-memory material.

With a bulk modulus K-4.9 = 78(3) GPa and K' = 5.1(4), beta-Pb3Bi2S6 is markedly less compressible than lillianite [K-0 = 44(2) GPa, K' = 7(1)]. Compressional anisotropy increases markedly in beta-Pb3Bi2S6 with compressibility along the b axis [M-0b = 130(6) GPa and M-b' = 19(3) in lillianite, M-4.9b = 145(4) GPa and M-b' = 16.0(7) in beta-Pb3Bi2S6] significantly larger than that along the other two axes [M-0a = 118(5) GPa, M-a' = 21(3), M-0c = 139(12) GPa, and M-c' = 31(10) in lillianite, M-4.9a = 242(12) GPa, M-a' = 8(1), M-4.9c = 242(5) GPa, and M-c' = 29(1) in beta-Pb3Bi2S6].

The behavior of lillianite at high pressure is an interesting case study in relation to non-quenchable ultrahigh-pressure phases likely occurring in the inner Earth, like post-perovskite MgSiO3, the oxide homologue N = 1 of the lillianite series. The beta-Pb3Bi2S6 structure, on the other hand, is the N = 3 homologue of the meneghinite series to which the higher-pressure modification of the post-perovskite structure also belongs (homologue N = 1). This makes the two forms of Pb3Bi2S6 potential equivalents of high- and ultrahigh-pressure Mg silicates that could occur both in the deep earth and in other rocky extrasolar planetary bodies.