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), and an intermediary member also exists [(Mg,Fe)CO₃]. Because all members of this group are isomorphous to one another and form multiple solid solution series within the group, some have classified the entire calcite group as one mineral, either called "Calcite Group" or Brownspar", with each one of its members as a variety.
See also Brownspar