Borate - Polymeric Ions

Polymeric Ions

A number of polymeric borate ions are known. They may be made by reacting B(OH)₃ or B₂O₃ with metal oxides. Examples include:

• diborate B₂O₅4− e.g. in Mg₂B₂O₅ (suanite)

• triborate B₃O₇5- in CaAlB₃O₇ (johachidolite)

• tetraborate B₄O₉6− in e.g. Li₆B₄O₉

• metaborates, such as LiBO₂ contain long chains of trigonal BO₃ structural units, each sharing two oxygen atoms with adjacent units.

• borates containing 3 and four coordinate boron, including the anion in borax.

• Borosilicate glass, also known as pyrex, is useful because it has a very low coefficient of thermal expansion and is resistant to cracking when heated, unlike soda glass. Structurally it is a silicate in which some SiO₄ units are replaced by BO₄, together with a cation to compensate for the difference in oxidation states of Si(IV) and B(III).