Borate minerals

(mineralogy) Any of the large and complex group of naturally occurring crystalline solids in which boron occurs in chemical combination with oxygen.

A large group of minerals in which boron is chemically bonded to oxygen. Boron is a fairly rare element. However, because of its chemical character, it is very susceptible to fractionation in earth processes and can become concentrated to a degree not found in other elements of similar abundance. Boron is symbolized B, carries atomic number 5, and has the ground-state electronic structure [He]2s²2p¹. The very high ionization potentials for boron mean that the total energy required to produce the B³⁺ ion is greater than the compensating structure energy of the resulting ionic solid, and hence bond formation involves covalent (rather than ionic) mechanisms. However, boron has only three electrons to contribute to covalent bonding involving four orbitals, (s, p_x, p_y, p_z). This results in boron being a strong electron-pair acceptor (that is, a strong Lewis acid) with a very high affinity for oxygen. The structural chemistry of boron and silicon (Si), when associated with oxygen (O), is quite similar. The BO₃, BO₄, and SiO₄ groups have a marked tendency to polymerize in the solid state, and this aspect of their behavior gives rise to the structural complexity of both groups. However, subtle differences in chemical bonding do give rise to differences in the character of this polymerization, particularly when water is also involved. These differences result in the very different properties of the resultant minerals and their very different behavior in earth processes. See also Boron.

Boron has an estimated primitive-mantle abundance of 0.6 part per million and a crustal abundance of 15 ppm. Despite this low abundance, fractionation in crustal processes results in concentration of boron to the extent that it forms an extensive array of minerals in which it is an essential constituent, and very complex deposits of borate minerals. Major concentrations of borate minerals occur in continental evaporite deposits (common in the desert regions of California and Nevada). Borate minerals are often very soluble in aqueous environments. In areas of internal drainage, saline lakes are formed, and continued evaporation leads to accumulation of large deposits of borate minerals. Borates may also occur in marine evaporites. Isolated-cluster borates are characteristic of metamorphosed boron-rich sediments and skarns. Most borosilicate minerals are characteristic of granitic-pegmatite environments, either as a pegmatite phase or as a constituent of their exocontact zone. In particular, tourmaline is the most widespread of the mixed-anion borate minerals, occurring in a wide variety of igneous, hydrothermal, and metamorphic rocks. See also Saline evaporites; Ore and mineral deposits; Pegmatite.

Despite its low crustal abundance, fractionation of boron in crustal processes leads to formation of deposits of borate minerals from which boron and (borates) can be easily extracted in large quantities. The easy availability and unique chemical properties result in boron being a major industrial material. It is widely used in soaps and washing powders. Boron combines well with silicon and other elements to form a wide variety of special-property glasses and ceramics; it also alloys with a variety of metals, producing lightweight alloys for specialty uses. Boron compounds usually have very low density; hence borates in particular are used as lightweight fillers in medicines, and also are used as insulation. The fibrous nature of some borate minerals results in their use in textile-grade fibers and lightweight fiber-strengthened materials. The mineral borax is used as a water softener and as a cleaning flux in welding and soldering. Boric acid has long been used as an antiseptic and a drying agent. Boron is also important as a constituent of inflammatory materials in fireworks and rocket fuel. Some mixed-anion borate minerals are used as gemstones. Tourmaline is of particular importance in this respect, forming pink (rubellite), blue-green (Paraiba tourmaline), green (chrome tourmaline), and pink + green (watermelon tourmaline) from a wide variety of localities. Kornerupine and sinhalite are also used as gemstones but are far less common. See also Boron; Tourmaline.