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