Structure and morphology of synthetic magnesium calcite

The X-ray crystal structure refinements of two synthetic calcite single crystals containing 3.1 and 5.7 mol% magnesium were carried out. The structure of the second crystal was also refined at -80°C. No significant structural difference from biogenic magnesium calcite was found. Magnesium within the crystals of calcite forms a solid solution over the compositional range investigated. The principal axes of vibration ellipsoids at -80°C resemble very closely those of calcite suggesting that the large thermal parameters in magnesium calcite reflect increased thermal vibrations more than positional disorder. The crystals of magnesium calcite express new rhombohedral faces due to the interaction of magnesium with growing crystals. The role of magnesium in the morphology of single crystal and globular aggregates of magnesium calcite is discussed in view of its possible relevance in both biomineralization and crystal growth.