The crystal structure of the aragonite group




LaCoste, Lucien Jean Batiste

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The aragonite group is a group of mineral carbonates consisting of aragonite (CaCO₃), cerussite (PbCO₃), witherite (BaCO₃) and strontianite (SrCO₃). All of these minerals exhibit the same external symmetry and are said to be isomorphous. A structure for aragonite has been proposed by Wyckoff and approximate values of the lattice constants have been given for the other members of the group, but no determination of the fine structure of cerussite, witherite, and strontianite has been made. This is probably due to the fact that each one contains both very heavy and very light atoms. A difficulty arises in the structure determination of such types of crystals because in general the contribution of the light atoms to the intensity of a diffraction line is negligible in comparison to that of the heavy atoms. The determination of the carbon and oxygen paramenters in these crystals is therefore a rather formidable undertaking. A solution of the structures of these crystals was therefore undertaken with the following fourfold purpose: first, to extend our knowledge of the fine structure of a group of crystals as a contribution to the data of x-ray crystallography, crystal physics, crystal chemistry and mineralogy; second, to obtain additional information concerning the crystallochemical properties of the CO₃ group, both for its interest in chemistry and for its aid in future structure analyses; third, to check the Zachariasen method of calculating atomic radii; and fourth, to provide a comparison of the structures of the members of an isomorphous series