Boron: Chemical information, Major minerals, Chemical reactions and Major uses

Chemical information

Boron exist as two natural stable isotopes, 11B (80.1%) and 10B (19.9%). The mass difference produces a wide range of δ 11B values (defined as a fractional difference between 11B and 10B and usually reported in parts per thousand), in natural waters ranging from 216 to 159. Thirteen isotopes of boron are known to exist. The shortest-lived isotope is 7 B with a half-life of 3.5 3 10222 s which decays through proton emission and alpha decay. Isotopic fractionation of B is controlled through the exchange reactions of the B species B(OH)3 and [B(OH)4] 2. Boron isotopes are also fractionated in the process of mineral crystallization, during H2O phase changes in hydrothermal systems, and during hydrothermal alteration of rocks. The basic physical and chemical properties of Boron are as follows:

The 10B isotope is suitable for capturing thermal neutrons. The nuclear industry enriches natural occurring boron to almost pure 10B. The less-valuable by-product, depleted boron, consists of almost pure 11B. Due to its high neutron cross-section, 10B is frequently used to control fission reactions in nuclear reactors as a neutron-capturing substance. Several industrial-scale enrichment methods have been established; but only the fractionated vacuum distillation of the dimethyl ether (methoxymethane, CH3OCH3) adduct of boron trifluoride (DME-BF3) and column chromatography of borates are being used.

Boron is the lightest element with an electron in the p-orbital (electron configuration is 1s2 2s2 2p1 ) in its ground state. It is the first element in Group 13 (aluminum family) of the periodic table but differs markedly from the other members of the group, which consists of aluminum, gallium, indium, and thallium. Boron is unique as it is the only nonmetal of the group and that it exhibits properties that are similar to its neighbor, carbon and its diagonal relative, silicon. Thus, like C and Si, boron forms covalent, molecular compounds. However, unlike those two, boron is more electron deficient and rarely obeys the octet rule. Given that it has three valence electrons, it typically forms trivalent neutral compounds such as BF3 wherein boron is surrounded by six electrons, assumes sp2 hybridization and, adopts a trigonal planar geometry. This confers electron-pair accepting (Lewis acidity) and multicenter bonding properties to boron.

Major minerals

The majority of the minerals containing boron are found in the borates class. However, a handful of rather rare minerals are found in the halide class, for example, avogadrite ((K,Cs)[BF4]), sulfates, for example, sturmanite (Ca6(Fe31,Al,Mn31)2(SO4)2[B(OH)4](OH)12· 25H2O).

Borates can, similar to C in organic chemistry and the silicates, form different polymeric groups, resulting in a rather large group of minerals. The best known of these include minerals such as: boracite (Mg3(B7O13)Cl), borax (Na2(B4O5)(OH)4· 8H2O), colemanite (Ca[B3O4(OH)3]· H2O) (Fig.1), hambergite (Be2(BO3)(OH)), inderite (MgB3O3(OH)5·5H2O), kernite (Na2[B4O6(OH)2]·3H2O), londonite ((Cs,K,Rb)Al4Be4(B,Be)12O28) (Fig. 2), tincalconite (Na2(B4O7)· 5H2O), and ulexite (NaCa[B5O6(OH)6]· 5H2O) (Fig. 3) (Box 3.4). In addition, there is a significant group of boron-containing silicates, which includes minerals such as: axinite- (Fe) (Ca2Fe21Al2BSi4O15OH), danburite (CaB2Si2O8) (Fig.4), datolite (CaB(SiO4)(OH)) (Fig. 5), dumortierite ((Al,Fe31)7(SiO4)3(BO3)O3), kornerupine (Mg3Al6(Si,Al,B)5O21(OH)), tourmaline group (e.g., dravite Na(Mg3)Al6(Si6O18)(BO3)3(OH)3(OH), elbaite Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)3(OH) (Fig.6), schorl Na(Fe21 3 )Al6(Si6O18)(BO3)3(OH)3(OH)), vesuvianite ((Ca,Na, )19(Al,Mg,Fe31)13( B, Al, Fe31)5(Si2O7)4(SiO4)10(OH, F, O)10), and wiluite (Ca19(Al, Mg)13(B, Al)5(Si2O7)4(SiO4)10(O,OH)10) (Fig. 7).

Chemical reactions

Crystalline boron is chemically inert and resistant to attack by boiling hydrofluoric or hydrochloric acid. It does react with hot concentrated nitric acid, hot sulfuric acid or hot mixture of sulfuric and chromic acids when finely divided.

The oxidation of boron in air is dependent on crystallinity, particle size, purity, and temperature. It does not react at room temperature but burns at higher temperature to form boron trioxide (B2O3).

Boron undergoes halogenation to give trihalides.

Major uses

Boron compounds are important in many industries, such as in making glass and detergents, and in agriculture with thousands of tons added to fertilizer each year as boron is essential to plant growth. The most important compounds are borax (sodium borate), boric oxide, and boric acid. The chief global industrial-scale use of boron compounds (nearly half of end-use) is in manufacturing of glass fiber for B-containing insulating and structural fiberglass, especially in Asia. Boron is added to the glass as borax pentahydrate or boron oxide, to affect the strength or fluxing qualities of the glass fibers. Boron fibers are high-strength, lightweight materials that are used primarily for advanced aerospace structures as a component of composite materials, as well as limited production consumer and sporting goods, for example, golf clubs and fishing rods.

Borax is used in various household laundry and cleaning products. It is also present in some tooth bleaching formulas. Sodium perborate serves as a source of active oxygen in many detergents, laundry detergents, cleaning products, and laundry bleaches. Boric acid is used as an insecticide, particularly against ants, fleas, and cockroaches. Borates are used as environmentally benign wood preservatives. The most important compounds of boron—boric (or boracic) acid, borax (sodium borate), and boric oxide —can be found in eye drops as mild antiseptics. Borax is also used as food preservative.

Boron is a useful dopant for such semiconductors as Si, Ge, and silicon carbide. Having one fewer valence electron than the host atom, it donates a hole resulting in p-type conductivity. Borates are used as flame retardants. Upon heating sodium borates release water from their crystalline structure, which help as a fire retardant.