Original paper

Lead-antimony sulfosalts from Tuscany (Italy). V. Definition and crystal structure of moeloite, Pb6Sb6S14(S3), a new mineral from the Ceragiola marble quarry

Orlandi, Paolo; Meerschaut, Alain; Palvadeau, Pierre; Merlino, Stefano

Abstract

Moëloite is a new lead-antimony sulfosalt, Pb6Sb6S14(S3), discovered in the Ceragiola marble quarry, near Seravezza town (Tuscany, Italy). Associated minerals are: sulphur, pyrite and enargite. It crystallizes as very thin needles and soft plumose aggregates of flexible thin fibres with a metallic lustre. Under the microscope it looks white, with moderate anisotropism, without polarization colours; reflectance values (air/oil): 40.0-37.8/26.6-24.2 (470 nm), 38.8-36.5/24.9-22.8 (546 nm), 38.6-36.7/23.4- 21.4 (589 nm), 36.5-34.7/21.2-20.1 (650 nm). Moëloite is orthorhombic, space group P21221, with unit cell parameters a 15.328(3), b 4.0400(8), c 23.054(5) Å; V 1427.6(5) Å3, with Z = 2; Dcalc. 5.86 g/cm3. Main X-ray powder diffraction lines [d(Å), I, hkl]: 3.724 (55)(106, 112), 3.427 (100) (403), 3.047 (85) (312, 214), 2.844 (58) (503), 2.779 (70) (016, 314), 2.753 (45) (116), 2.017 (80) (614, 020), 1.733 (50) (0112, 618, 2013). The crystal structure reveals Pb6Sb6S16 rods parallel to b, and connected along c by a (S3)2- trimer due to an additional S atom, to form (100) rod-layers. The crystal structural formula is Pb2+6Sb3+6S2-14(S3)2-. The formation of moëloite is subordinated to a high sulfur fugacity, which explains the S excess of its formula. The name is in honour of Dr. Y. Moëlo (Nantes, France), who first synthesized this compound.

Keywords

moeloitesulfosaltleadantimonycrystal structuretuscanyitaly