**1. Introduction**

Galenobismutite PbBi2S4 is a Bi-sulfosalt usually found in hydrothermal veins or associated with fumarolic deposits [1,2]. Like other Bi-minerals, it has an important role in the reconstruction of the formation of ore deposits as it is sensitive to physical–chemical fluctuations and can constrain the genesis of ore.

According to sulfosalt classification [3] galenobismutite is classified among commensurate composite derivatives of cannizzarite in a sub-group with angelaite, Cu2AgPbBiS4 [4], nuffieldite, Cu1.4Pb2.4Bi2.4Sb0.2S7 [5] and weibullite, Ag0.33Pb5.33Bi8.33(S,Se)18 [6]. It has a distinctly different crystal structure from the chemically (stoichiometrically) similar berthierite FeSb2S4 [7], garavellite FeSbBiS4 [8] and clerite MnSb2S4 [9] which form a berthierite isotypic series [3].

Chemical substitution of Sb for Bi and Fe for Pb are common in gelenobismutite. It is illustrated by galenobismutite from Beiya porphyry- and skarn-type deposits that contain Sb up to 0.39% and Fe up to 0.42% [10]. Selenium can replace sulfur in galenobismutite. In galenobismutite from Vulcano Island (Italy) [1] heterogeneous distribution of selenium in the Sulphur sites was found, with a total amount

of up to 0.13 atoms of Se per formula unit. Moreover, galenobismutite from Vulcano shows an unusual presence of Cl, according to the coupled heterovalent substitution scheme: Pb2<sup>+</sup> + Cl<sup>−</sup> = Bi3<sup>+</sup> + S2<sup>−</sup> [2].

The crystal structure of galenobismutite is orthorhombic, space group *Pnam*, and was described firstly by Wickman [11], then by Iitaka and Nowacki [12], and later classified by Makovicky [13] as being representative of a specific subgroup of cannizzarite-type structures.

The crystal structure contains three cation positions. M1 has a slightly distorted octahedral coordination and forms fragments of galena-like structure two octahedra wide by sharing edges with a conjugated M1 octahedron. M2 is surrounded by seven S atoms forming a "lying" (prism axis is perpendicular to the *c* crystal axis) mono-capped trigonal prism. M3 polyhedron is a "standing" (prism axis parallel to the *c* crystal axis) bi-capped trigonal prism (CN8) with one of the capping ligands relatively distant, so the coordination can also be described as a 7+1 (Figure 1).

The determination of the distribution of Bi and Pb in the three sites by refinement of X-ray diffraction data is practically impossible due to the similar number of electrons (83 and 82, respectively). It is therefore based on the bond valence calculations. According to Pinto et al. [1], who used bond lengths, bond valences and geometrical characteristics of the coordination polyhedral to interpret the occupancy on the M1, M2 and M3 positions. M1 is considered fully occupied by Bi, while M2 and M3 are mixed sites dominated by Bi and Pb, respectively.

The configuration of this structure, isotypic with calcium ferrite (CF) CaFe2O4, is of a particular interest for high pressure mineral physics. Finger and Hazen [14] include among the seven structure-types with exclusively six-coordinated silicon NaAlSiO4 [15,16], the analogue of CF. This structure bears a close relationship to hollandite, another VISi -coordinated structure type. Both structures consist of double octahedral chains which are joined to form 'tunnels' parallel to *c* that accommodate the alkali or alkaline-earth cations. In hollandite four double chains form square tunnels, whereas in CF four double chains define triangular tunnels.

Akaogi et al. [17] were the first who synthesized NaAlSiO4 with CF structure at pressures higher than 18–20 GPa. Tutti et al. [18] found that this phase is stable at pressure up to at least 70–75 GPa and temperatures 800–2200 ◦C indicating it as an important carrier of Na and Al in the lower Mantle.

The most regular CF-type structure known is that of PbSc2S4 ([19], Figure 1b). The reported crystal structure of NaAlSiO4 ([16], Figure 1c), obtained from a powder sample, has a substantially more distorted octahedral coordination. Dubrovinsky et al. [20] reported the structure of NaAlSiO4 at 35 GPa, likewise done on a powder sample (Figure 1d). The data suggest that the M1 coordination becomes more regular without significant contraction, whereas M2 and Na coordinations significantly contract keeping their general shapes. Compared to the other CF structures, galenobismutite differs in having CN7 coordination of the M2 site and a significantly distorted coordination of the M3 site. M1 coordination is eccentric, unlike in PbSc2S4 or NaAlSiO4. The increased CN of M2 and distortions of the other two coordinations in galenobismutite are explained as a stereochemical effect of the lone electron pair of Bi3<sup>+</sup> [21].

The first high-pressure study of galenobismutite was done by Olsen et al [21] at pressures up to 8.9 GPa with single crystal X-ray diffraction. They found a bulk modulus of *K*<sup>0</sup> equal to 43.9(7) GPa and a *K*' of 6.9 (3). No phase transition was observed in this pressure range and, interestingly, although the stereochemical activity of Lone Electron Pair (LEP)'s decreased with pressure, the structure did not approach the CF isotype but moved further away from its typical configuration, keeping its distinct character [21].

The present paper extends the study of the baric behavior of galenobismutite over a significantly larger pressure range, up to 20.9 GPa, by a synchrotron single crystal X-ray diffraction study in order to obtain a more complete picture of its behavior under high pressure. Really the relevance of high pressure single crystal X-ray diffraction data with respect those from high pressure powder diffraction was very recently highlight in several papers i.e. [22,23].

**Figure 1.** Crystal structures of: (**a**) galenobismutite, projected along [001] with x axis on the vertical and *y* axis on the horizontal line; (**b**) PbSc2S4 [19]; (**c**) NaAlSiO4 at room pressure [16]; (**d**) NaAlSiO4 at 35 GPa [20].
