Borate minerals

The borate minerals are minerals which contain a borate anion group. The borate (BO₃) units may be polymerised similar to the SiO₄ unit of the silicate mineral class. This results in B₂O₅, B₃O₆, B₂O₄ anions as well as more complex structures which include hydroxide or halogen anions. The [B(O,OH)₄]⁻ anion exists too.

Many borate minerals, such as borax, colemanite, and ulexite, are salts: soft, readily soluble, and found in evaporite contexts. However, some, such as boracite, are hard and resistant to weathering, more similar to the silicates.

There are over 100 different borate minerals. Borate minerals include:

Nickel–Strunz Classification -06- Borates

IMA-CNMNC proposes a new hierarchical scheme (Mills et al., 2009). This list uses it to modify the Classification of Nickel–Strunz (mindat.org, 10 ed, pending publication).

Borate

Borates are the name for a large number of boron-containing oxyanions. The term "borates" may also refer to tetrahedral boron anions, or more loosely to chemical compounds which contain borate anions of either description. Larger borates are composed of trigonal planar BO₃ or tetrahedral BO₄ structural units, joined together via shared oxygen atoms and may be cyclic or linear in structure. Boron most often occurs in nature as borates, such as borate minerals and borosilicates.

Structures

The simplest borate anion, the orthoborate ion, BO₃³⁻ is known in the solid state, for example in Ca₃(BO₃)₂. In this it adopts a near trigonal planar structure. It is a structural analogue of the carbonate anion CO₃²⁻, with which it is isoelectronic. Simple bonding theories point to the trigonal planar structure. In terms of valence bond theory the bonds are formed by using sp²hybrid orbitals on boron. Some compounds termed orthoborates do not necessarily contain the trigonal planar ion, for example gadolinium orthoborate, GdBO₃ contains the polyborate (B₃O₉)⁹⁻ ion, whereas the high temperature form contains planar BO₃³⁻.