The Borate minerals are more complex in their structures than typical carbonates,
but because of the scarcity and limited distribution of boron in the Earth's
crust there are only a few borates that can be considered common.
Because boron in the BO3 ionic group has a positive three
(+3) charge, it only requires half of the bond strength of each negative
two (-2) oxygen. This allows the oxygens to bond with other borons evenly
and thus link boron groups together into compound groups, chains, sheets
and even a framework structure using BO4 tetrahedrons
linked to BO3 groups. This makes the borates
similar to the many structural variations found in the Silicate Class of minerals.
These structural variations explain the large size of this subclass in terms of numbers of species.
The chemistry is certainly not the culprit.
There is little variation in the chemistry of these minerals as a quick scan of the list below reveals mostly sodium, calcium and/or magnesium borates, many with hydroxides, many hydrated and some with chlorine; but little else in terms of chemical variation.