DETERMINATION AND REFINEMENT OF CRYSTAL STRUCTURE OF MARGAROSANITE PBCA2SI3O9

The crystal structure of margarosanite with a = 6.768 A ±.004, b = 9.575 Å ±.004, c = 6.718 A ±.005, α = 110.36° ±.03, β = 102.98° ±.04, γ = 83.02 ±.05, and with space group PI was solved by application of the minimum function to P(xyz), and the use of heavy-atom techniques. This procedure yielded a trial structure which was refined by a least-squares method using the full matrix. The unit cell ideally contains 2 (PbCa2Si3O9). The margarosanite structure has the following principle features: 1. planes of tetrahedral sites alternate with planes of calcium sites between sheets of close-packed oxygen atoms parallel to (101); 2. Pb and Ca are ordered; 3. Ca(l) polyhedra form an infinite edge-sharing chain parallel to [101] and Pb and Ca(2) sites alternate along the edge of this chain; 4. tetrahedral sites form three-membered rings which are linked to the Ca(l) polyhedral chain; and 5. the two Ca atoms have 6-fold coordination and the Pb atom has 7-fold coordination. © 1969, Walter de Gruyter. All rights reserved.