Powder Diffraction

Table of Contents - December 2010 - Volume 25, Issue 04  

Technical Articles

Crystal structure of the mineral strontiodresserite from laboratory powder diffraction data

P. S. Whitfielda1 c1, L. D. Mitchella2, Y. Le Pagea3, J. Margesona4 and A. C. Robertsa5

a1 Institute for Chemical Process and Environmental Technology, National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada

a2 Institute for Research in Construction, National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada

a3 Institute for Chemical Process and Environmental Technology, National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada

a4 Institute for Research in Construction, National Research Council Canada, 1200 Montreal Road, Ottawa, Ontario K1A 0R6, Canada

a5 Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

Abstract

The crystal structure of the mineral strontiodresserite, (Sr,Ca)Al2(CO3)2(OH)4⋅H2O, from the Francon Quarry, Montreal, Quebec, Canada, has been solved from laboratory powder diffraction data using a combination of charge-flipping and simulated annealing methods. The structure is orthorhombic in space group Pnma with a=16.0990(7), b=5.6133(3), and c=9.1804(4) Å (Z=4) and the framework of the mineral is isostructural with that of dundasite. The strontium has a coordination number of 9 and the carbonate anions form a bridge between the SrO9 polyhedra and AlO6 octahedra. The water molecule lies in a channel that runs parallel to the b axis. An ordered network of hydrogen atoms could be uniquely determined from crystal-chemical principles in the channels of strontiodresserite. Ab initio density functional theory (DFT) energy minimization of the whole structure gave results in full agreement with X-ray refinement results for nonhydrogen atoms. The stability of this model (as well as that of the corresponding model of dundasite) in the proposed Pnma space group was tested by DFT optimization in space group P1 of random small distortions of this structure. This test confirms that both minerals are isostructural, including their hydrogen-bond networks.

(Received June 09 2010)

(Accepted September 16 2010)

Key words

  • mineral;
  • charge flipping;
  • simulated annealing;
  • Rietveld refinement

Correspondence:

c1 Author to whom correspondence should be addressed. Electronic mail: pamela.whitfield@nrc.gc.ca