Calcite group - Group of minerals belonging to the carbonate group that are isomorphous with one another and have the same properties, such as that they all:

Crystallize in the trigonal sect of the hexagonal system, and most commonly form rhombohedrons and scalenohedrons.
Have perfect rhombohedral cleavage
Exhibit a very strong double refraction in transparent rhombohedrons

Minerals in this group can partially or fully replace one another. For example, iron may take the place of magnesium in Magnesite (MgCO₃), transforming it to Siderite (FeCO₃).