7
Boron [B, 5]

7.1 INTRODUCTION
Boron (B), a metalloid of the group 13 in the periodic table of elements, occurs in
the Earth’s crust at an average amount of 10 mg/kg. It is likely to be concentrated
to about 30 mg/kg in acidic igneous rocks. In sedimentary rocks, its contents are
higher than in igneous rocks, and are closely associated with the clay fraction; for
example, in quaternary clay, its amounts are up to 96 mg/kg. The highest B contents,
up to 145 mg/kg, are in limestones. It may also be concentrated in some carboniferous sediments and coals, as well as in fly ash, where its mean content is estimated at
509 mg/kg (Llorens et al. 2000).
Boron is widely distributed in the hydrosphere, and its water solubility highly
controls its distribution in the environment. The largest quantities of B are accumulated in marine evaporites and marine argillaceos sediments. Borate minerals
are usually deposited as evaporates (e.g., borax and kernite). Boron may also be
increased in some volcanic emissions, mainly as orthoboric acid, H 3 BO 3 , and thus it
occurs in higher amounts in volcanic rocks.
Boron oxidation state is +3. It is a constituent of several minerals, and some of its
common minerals are borax, Na 2 B 4 O 7 ·10H 2 O; colemanite, Ca 2 B 6 O 11 ·5H 2 O; ulexite,
NaCaB 5 O 9 ·8H 2 O; kernite, Na 2 B 4 O 6 ·3H 2 O; and tourmaline, NaFe 2+ 3Al 6 (BO 3 )·3Si 6
O 18 (OH) 4 . Boron is often associated with feldspars and micas.
World (without the United States) production of B, mainly as boric oxide (B 2 O 3 ),
in 2010 was 3500 kt (rounded), of which 1200 kt (18% of the global B demand) was
produced in Turkey, as colemanite (USGS 2011). The United States supplies about
70% of the global B demand.
The major industrial-scale uses of B compounds are in the production of fiberglass
and heat-resistant borosilicate glass and ceramics. Addition of B increases resistance
to the thermal shock of various products. It is used in the synthesis of organic chemicals. Boron is widely used in fertilizers and pesticides, cosmetics, antiseptics, leather
tanning, and laundry products. Also, a few boron-containing organic pharmaceuticals are produced. Borax (Na 2 B 4 O 7 ·10H 2 O) is a commonly used compound. Boron is
a good neutron absorber and is used to control nuclear reactions.
7.2 SOILS
Boron contents in soils vary between 10 and 100 mg/kg, being enriched in calcisols
(Table 7.1). World B average for soils is estimated at 42 mg/kg. Average B contents
in various soils of the United States are within the range 20–55 mg/kg. A high B
content, up to 1622  mg/kg (average 65  mg/kg), is reported for some soils in the
Slovak Republic (Čurlik and Šefčik 1999). Boron is inherited mainly from the parent
rocks, but industrial pollution, sludge, coal combustion, fly ash, and B fertilizers are
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