Single-crystal X-ray structure refinements of two biogenic magnesian calcite crystals

Structural parameters have been refined for two single crystals of biogenic magnesian calcite containing 6.4 and 12.9 mol% MgCO₃, respectively, and yielding weighted residuals of 0.027 and 0.035. Combined with existing data for pure calcite, the M²⁺-O bond length and the volume of the M²⁺ (M²⁺ = Ca,Mg) coordination octahedron decrease linearly with Mg content. No evidence for cation ordering of the type found in dolomite was found. Equivalent isotropic temperature factors calculated for the M²⁺, C, and O sites increase approximately linearly with Mg content and are significantly larger than in calcite, magnesite, and dolomite. -from Authors