Il museo di Mineralogia

Vengono riportati i tratti principali del museo di mineralogia dell'Università di Pavia, dalle sue origini nel 1771 nell'ambito del Museo di Storia Naturale di Pavia fino ai giorni nostri. Vengono descritte le principali collezioni di minerali conservate e un cenno viene fatto anche per la piccola collezione di meteoriti che contiene un pezzo di grande interesse storico-scientifico

Aschamalmite (Pb6Bi2S9): crystal structure and ordering scheme for Pb and Bi atoms

The first single-crystal structure refinement of aschamalmite (Pb6Bi2S9) from Susa Valley (Piedmont, Italy) is reported. The mineral is monoclinic, C2/m, a = 13.719(1) Å, b = 4.132(1) Å, c = 31.419(3) Å, β = 90.94(1)°, V = 1 780.8(4) Å3, Z = 4. The Pb6Bi2S9 compound crystallizes also in an orthorhombic form as heyrovskyite (Cmcm) and our study is focused on understanding the reason leading to a change in symmetry. The aschamalmite structure forms because of ordering between Pb and Bi on the margins of the two octahedral layers that are symmetrically equivalent in heyrovskyite. The two alternate set of octahedral slabs are not related by a crystallographic mirror plane and the symmetry decreases to monoclinic. The cation ordering couples opposite sequences of Pb and Bi octahedra at the margins of slabs. In particular, the succession [Me4A]Bi-[Me5A]Pb-[Me4A]Bi-[Me5A]Pb faced to the series [Me4B]Pb-[Me5B]Bi-[Me4B]Pb-[Me5B]Bi occurs in about 70% of the unit-cells of the crystal, while the contrary sequence ([Me4A]Pb-[Me5A]Bi-[Me4A]Pb-[Me5A]Bi faced to [Me4B]Bi-[Me5B]Pb-[Me4B]Bi-[Me5B]Pb) occurs in the remaining unit-cells. The marginal octahedra have ideal populations (a.p.f.u.): [Me4A]1.40Bi+0.60Pb, [Me4B]1.40Pb+0.60Bi, [Me5A]1.40Pb+0.60Bi, [Me5B]1.40Bi+0.60Pb, in agreement with our structure-refinement results. The probable site populations for pure heyrovskyite have been proposed, as well as the reasons that prevent the formation of a completely ordered monoclinic phase

Crystal structure refinement of margarosanite PbCa2Si3O9 and relationship with walstromite BaCa2Si3O9

X-ray single-crystal diffraction data of margarosanite from the Parker Shaft, North Mine Hill, Franklin, New Jersey were collected using a Bruker-AXS three-circle diffractometer equipped with the Smart-Apex CCD detector. Margarosanite is triclinic, P-1, with unit cell parameters: a = 6.7370(3) Å, b = 9.5295(4) Å, c = 6.6961(3) Å, alfa = 110.185(2)°, beta = 103.048(2)°, gamma = 83.037(2)°, V = 392.63(3) Å3, Z = 2. The new structure refinement performed on 9206 diffraction data in the 2θ range 4 < θ < 107° (R1 = 0.035) improves the crystallographic details of a crystal structure described in the seventies and not further studied up to now. The crystal structure of margarosanite is defined by two different structural layers parallel to (-1 0 1): a Si layer constituted by isolated 3-fold rings of SiO4 tetrahedra and a Ca layer made by dimers of CaO6 octahedra and chains of CaO8 polyhedra interconnected via edge-sharing to form a Ca-polyhedral sheet. Additional edge-sharing chains of distorted 8-fold coordinated Pb polyhedra are also present. The crystallographic comparison between margarosanite and walstromite, the Ba-bearing natural counterpart, has been carried out in order to emphasize that the two minerals are isostructural