Tubulite, ~Ag2Pb22Sb20S53, a new Pb-Ag-Sb sulfosalt from Le Rivet quarry, Peyrebrune ore field (Tarn, France) and Biò, Borgofranco mines, Borgofranco d'Ivrea (Piedmont, Italy)

Abstract

Tubulite, ~ Ag 2 Pb 22 Sb 20 S 53 , is a new Pb–Ag–Sb sulfosalt discovered at Le Rivet quarry, Peyrebrune ore field (Tarn, France) and Bio, Borgofranco mines, Borgofranco d’Ivrea (Piedmont, Italy). As indicated by the name, it forms very thin perfect micro-tubes, 100 to 600 µm in length, 40 to 100 μm in diameter, and only 1 to 2 μm in thickness; a hair-like variety is also present at Bio. At Le Rivet, it is associated with galena, pyrite, pyrrhotite, arsenopyrite, stibnite, and various Sb sulfosalts of Pb, Ag and Cu, in a gangue of quartz, baryte and carbonates. At Bio, it is associated with galena, sphalerite, chalcopyrite, pyrite, marcasite, and various Sb sulfosalts of Pb, Ag and Cu, within the same type of gangue. Tubulite is metallic black; optical properties could not be observed under the microscope, due to the crystal morphology. Electron microprobe analysis gave (wt.% – average of 8 anal.): Ag 2.7(2), Pb 46.6(9), Sb 26.1(8), S 17.8(5), Total 93.2(2.1). The low total is due to the thinness of the tube wall. According to crystallographic study (electron and X-ray diffraction), tubulite is monoclinic (space group Pc , P 2/ c or P 2 1 / c) with unit-cell parameters a 4.132(2), b 43.1(2), c 27.4(1) A, β 93.2°, V4872(40) A 3 , with Z = 2. Weak reflections in the [010] electron diffraction pattern indicate a 2a superstructure. Main diffraction lines are [d (A), ( I )]: 3.99 ( 35 ), 3.69 ( 60 ), 3.36 ( 100 ), 3.28 ( 55 ), 2.99 ( 55 ), 2.912 ( 55 ), 2.063 ( 75 ). The unit cell of tubulite is very close to those of sterryite and parasterryite; like these sulfosalts, tubulite is probably an expanded derivative of owyheeite. Its peculiar tubular morphology is discussed within the general framework of crystals whose habit presents a circular symmetry. It is proposed that micro-tubes were initiated by capillary forces acting on very thin lamellar crystallites around gas bubbles or liquid droplets.