The Dolomite Group of Minerals
The Dolomite Group is composed of minerals with an unusual
trigonal bar 3 symmetry.
The general formula of this group is AB(CO3)2,
where A can be either calcium, barium and/or strontium and the B can be either iron, magnesium, zinc and/or manganese.
The structure of the Dolomite Group is taken from the Calcite Group structure.
The Calcite Group structure is layered with alternating carbonate layers and metal ion layers.
The structure of the Dolomite Group minerals is layered in such a way that the
A metal ions occupy one layer which is followed by a carbonate layer which is followed by the B metal ion layer followed by another carbonate (CO3) layer, etc.
The layering looks like this: |A|CO3|B|CO3|A|CO3|B|CO3|A|...
This ordered layering of different or nonequivalent ions causes a loss of the two fold rotational axes and mirror planes that are present in the Calcite Group structure.
Dolomite's symmetry class is bar 3 whereas the Calcite Group's symmetry class is bar 3 2/m.
The loss of symmetry allows only simple crystal forms to be used by the Dolomite Group minerals, mostly rhombohedrons.
Dolomite is a very common mineral and ankerite is much more scarce.
The other members are considered rare to very rare.
The rarity of the members of this group can be tied to the closeness in radius of the
A and B ions.
In dolomite the A and B ions are calcium and magnesium which have the largest ionic radius differential of the group (approximately 33%).
If the A and B ions are close in radius, then they tend to not segregate as easily into the separate
A and B layers, which is required to form this structure and therefore these minerals.
MINERALS THAT BELONG TO THE DOLOMITE GROUP:
Ca(Fe, Mg, Mn)(CO3)2
- Benstonite (Ba, Sr)6(Ca, Mn)6Mg(CO3)13
- Huntite CaMg3(CO3)4
- Kutnohorite Ca(Mn, Mg, Fe)(CO3)2
- Minrecordite CaZn(CO3)2
- Norsethite BaMg(CO3)2
The borate minerals nordenskoldine
are isostructural with the Dolomite Group minerals.