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.