**Paola Comodi 1,\*, Azzurra Zucchini 1, Tonci Bali´c-Žuni´c 2, Michael Hanfland <sup>3</sup> and Ines Collings <sup>3</sup>**


Received: 27 March 2019; Accepted: 16 April 2019; Published: 18 April 2019

**Abstract:** High-pressure single-crystal synchrotron X-ray diffraction data for galenobismutite, PbBi2S4 collected up to 20.9 GPa, were fitted by a third-order Birch-Murnaghan equation of state, as suggested by a *FE*-*fE* plot, yielding *V*<sup>0</sup> = 697.4(8) Å3, *K*<sup>0</sup> = 51(1) GPa and *K*' = 5.0(2). The axial moduli were M0a = 115(7) GPa and Ma' = 28(2) for the *a* axis, M0b = 162(3) GPa and Mb' = 8(3) for the *b* axis, M0c = 142(8) GPa and Mc' = 26(2) for the *c* axis, with refined values of *a0*, *b0*, c0 equal to 11.791(7) Å, 14.540(6) Å 4.076(3) Å, respectively, and a ratio equal to M0a:M0b:M0c = 1.55:1:1.79. The main structural changes on compression were the M2 and M3 (occupied by Bi, Pb) movements toward the centers of their respective trigonal prism bodies and M3 changes towards CN8. The M1 site, occupied solely by Bi, regularizes the octahedral form with CN6. The eccentricities of all cation sites decreased with compression testifying for a decrease in stereochemical expression of lone electron pairs. Galenobismutite is isostructural with calcium ferrite CaFe2O4, the suggested high pressure structure can host Na and Al in the lower mantle. The study indicates that pressure enables the incorporation of other elements in this structure, increasing its potential significance for mantle mineralogy.

**Keywords:** galenobismutite; high pressure; single-crystal X-ray synchrotron diffraction; equation of state; calcium ferrite structure type; lone electron pair
