isotope ratio is also an important consideration in geochemistry because, for marine
carbonates, it is used as a paleo-pH recorder for ancient seawater [5–9]. It is also
known that sound absorption in the ocean (∼1 kHz) involves chemical equilibria
with relaxation rates that correspond to the boric acid-borate equilibria [10].
Important information on the structure of these ions can be obtained by solubility
and pH measurements on solutions, X-ray diffraction study of crystals, and nuclear
magnetic resonance.
The solubilities of the simpler water-boron oxide system was investigated by
Kracek et al. [11]. They identified the solid compounds ice, H 3 BO 3 , three modifications of HBO 2 , and crystalline B 2 O 3 . The solubility curves of boric acid and
several sodium borates were reported by Blasdale and Slansky soon thereafter [12].
The solid phases investigated were (ortho)boric acid, H 3 BO 3 ; three hydrated forms
of sodium tetraborate, Na 2 B 4 O 7 ⋅ nH 2 O, with n = 4 (the mineral kernite), 5, 10;
sodium pentaborate pentahydrate, NaB 5 O 8 ⋅ 5H 2 O; and two hydrated forms of
sodium metaborate, NaBO 2 ⋅ nH 2 O, with n = 2, 4. An alternate way to denote a
series of stoichiometries, common in geochemistry, is with respect to the ratio of
oxides lNa 2 O: mB 2 O 3 : nH 2 O, or shortened as (l:m:n).
The crystal structures of boron oxide and its hydrates ortho- and metaboric acid
are given in Table 1. Trigonal boron oxide, originally thought to contain tetrahedral
boron [13], actually consists of a 3D-network of corner-linked BO 3 units [14–16].
The high-pressure orthorhombic form does consist of fused 6-membered rings of
BO 4 tetrahedra [17]. Orthoboric acid was originally assumed to have the hydrogen
atoms halfway between the oxygens [18]. It was also shown that there could be
disorder in the layering by electron diffraction [19]. Refinement shows that both
polytypes (AB [20] or ABC [21] stacking) of orthoboric acid (0:1:3) consist of
stacks of approximately planar layers of hydrogen-bonded B(OH) 3 molecules with
approximate C 3h symmetry. Metaboric acid (0:1:1) exists in at least three forms,
two of which were discovered by Tazaki [22]. The orthorhombic α-form consists of
sheets [23] of B 3 O 3 (OH) 3 with approximately C s symmetry, held together by
hydrogen bonds [24]. The monoclinic β-form was shown to contain BO 4 tetrahedra
and planar B 2 O 5 groups [25], and further refinement showed that it consists of
endless zigzag chains of [B 3 O 4 (OH)(OH 2 )] [26, 27]. In essence, the H 3 B 3 O 6
molecules have condensed together and the water molecule produced has bonded to
one of the two boron condensation sites. The cubic γ-form contains only tetrahedral
boron atoms [27, 28].
The crystal structures of some sodium and lithium borates are given in Table 2.
Anhydrous sodium borate (3:1:0) consists of discrete BO 3
3− ions [29]. The
hydrated sodium borate (2:1:1) consists of discrete [BO 2 (OH)]
2− ions connecting
sheets of edge-shared NaO polyhedra [30]. There are several hydrates of sodium
metaborate (1:1:n), but only two of them consist of the tetrahedral [B(OH) 4 ]
− anion,
the octahydrate [31] and the tetrahydrate [32]. There are also two monomeric
lithium salts (n = 16,8) [33–36]. The crystal structure of polyborates will be discussed in a separate paper.
144
C. C. Pye
carbonates, it is used as a paleo-pH recorder for ancient seawater [5–9]. It is also
known that sound absorption in the ocean (∼1 kHz) involves chemical equilibria
with relaxation rates that correspond to the boric acid-borate equilibria [10].
Important information on the structure of these ions can be obtained by solubility
and pH measurements on solutions, X-ray diffraction study of crystals, and nuclear
magnetic resonance.
The solubilities of the simpler water-boron oxide system was investigated by
Kracek et al. [11]. They identified the solid compounds ice, H 3 BO 3 , three modifications of HBO 2 , and crystalline B 2 O 3 . The solubility curves of boric acid and
several sodium borates were reported by Blasdale and Slansky soon thereafter [12].
The solid phases investigated were (ortho)boric acid, H 3 BO 3 ; three hydrated forms
of sodium tetraborate, Na 2 B 4 O 7 ⋅ nH 2 O, with n = 4 (the mineral kernite), 5, 10;
sodium pentaborate pentahydrate, NaB 5 O 8 ⋅ 5H 2 O; and two hydrated forms of
sodium metaborate, NaBO 2 ⋅ nH 2 O, with n = 2, 4. An alternate way to denote a
series of stoichiometries, common in geochemistry, is with respect to the ratio of
oxides lNa 2 O: mB 2 O 3 : nH 2 O, or shortened as (l:m:n).
The crystal structures of boron oxide and its hydrates ortho- and metaboric acid
are given in Table 1. Trigonal boron oxide, originally thought to contain tetrahedral
boron [13], actually consists of a 3D-network of corner-linked BO 3 units [14–16].
The high-pressure orthorhombic form does consist of fused 6-membered rings of
BO 4 tetrahedra [17]. Orthoboric acid was originally assumed to have the hydrogen
atoms halfway between the oxygens [18]. It was also shown that there could be
disorder in the layering by electron diffraction [19]. Refinement shows that both
polytypes (AB [20] or ABC [21] stacking) of orthoboric acid (0:1:3) consist of
stacks of approximately planar layers of hydrogen-bonded B(OH) 3 molecules with
approximate C 3h symmetry. Metaboric acid (0:1:1) exists in at least three forms,
two of which were discovered by Tazaki [22]. The orthorhombic α-form consists of
sheets [23] of B 3 O 3 (OH) 3 with approximately C s symmetry, held together by
hydrogen bonds [24]. The monoclinic β-form was shown to contain BO 4 tetrahedra
and planar B 2 O 5 groups [25], and further refinement showed that it consists of
endless zigzag chains of [B 3 O 4 (OH)(OH 2 )] [26, 27]. In essence, the H 3 B 3 O 6
molecules have condensed together and the water molecule produced has bonded to
one of the two boron condensation sites. The cubic γ-form contains only tetrahedral
boron atoms [27, 28].
The crystal structures of some sodium and lithium borates are given in Table 2.
Anhydrous sodium borate (3:1:0) consists of discrete BO 3
3− ions [29]. The
hydrated sodium borate (2:1:1) consists of discrete [BO 2 (OH)]
2− ions connecting
sheets of edge-shared NaO polyhedra [30]. There are several hydrates of sodium
metaborate (1:1:n), but only two of them consist of the tetrahedral [B(OH) 4 ]
− anion,
the octahydrate [31] and the tetrahydrate [32]. There are also two monomeric
lithium salts (n = 16,8) [33–36]. The crystal structure of polyborates will be discussed in a separate paper.
144
C. C. Pye