Glossary of Terms
Group of related minerals belonging to the carbonate group that are isomorphous with one another and have the following specific properties:
They all crystallize in the trigonal division of the hexagonal crystal system, and most commonly form rhombohedrons and scalenohedrons.
They have perfect rhombohedral cleavage and exhibit 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).