Normally white or colorless, it has a rhombohedral or hexagonal symmetry and a highly ordered arrangement of cations, which distinguishes it from a randomly substituted calcite.  
It can often be found in ultrabasic igneous rocks, such as serpentitites and carbonatites, as well as in hydrothermal veins and metamorphosed carbonate sediments.  Normally white or colorless, it has a rhombohedral or hexagonal symmetry and a highly ordered arrangement of cations, which distinguishes it from a randomly substituted calcite.  It is commonly found, however, in limestone and in the rock which shares its name.  FTP, identify this white or colorless carbonate mineral with chemical formula CaMg(CO3)2, which shares its name with a range of the Alps in northeastern Italy.
It can often be found in ultrabasic igneous rocks, such as serpentitites and carbonatites, as well as in hydrothermal veins and metamorphosed carbonate sediments.  
dolomite