Original paper

Crystal structure refinement of margarosanite PbCa2Si3O9 and relationship with walstromite BaCa2Si3O9

Callegari, Athos Maria; Boiocchi, Massimo

Abstract

X-ray single-crystal diffraction data of margarosanite, with the formula (Pb0.96 Ca0.04)Σ1 (Ca1.93 Mn2+ 0.04Fe 2+ 0.02Pb0.01)Σ2Si3 O9, 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, P1 , with unit cell parameters: a = 6.7370(3) Å, b = 9.5295(4) Å, c = 6.6961(3) Å, α = 110.185(2)°, β = 103.048(2)°, γ = 83.037(2)°, V = 392.63(3) Å3 , Z = 2. The new structure refinement performed on 9206 diffraction data in the 2 range 4 <  < 107° (R 1 = 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.

Keywords

isostructuralcyclosilicatescrystal-structure refinementmargarosanitewalstromite