14.8: Borate Minerals

Anhydrous Borate Group
boracite Mg₃ClB₇O₁₃
sinhalite MgAlBO₄

Hydrous Borate Group
borax Na₂B₄O₅(OH)₄•8H₂O
kernite Na₂B₄O₆(OH)₂•3H₂O
ulexite NaCaB₅O₆(OH)₆•5H₂O
colemanite CaB₃O₄(OH)₃•H₂O
dumortierite Al_6-7BSi₃O₁₅(O,OH)₃

Mineralogists have identified many borate minerals. Most, especially the anhydrous borates, are very rare. Borate minerals have complex structures and chemistries, due in large part to the small size and trivalent nature of ionic boron. They have structural similarity to carbonates and nitrates because boron combines with oxygen to form anionic groups: (BO₃)^3- or (BO₄)^5-.

Borates have high solubilities in water and, consequently, are only found in relatively arid environments. They are deposited in evaporite deposits which are sometimes mined to produce boron.

Borax Na₂B₄O₅(OH)₄•8H₂O

Origin of Name
From the Persian word burah, meaning “white.”

Figure 14.396: Borax from Boron, California; the sample is 30 cm across

Figure 14.397: Borax from Boron, California

Hand Specimen Identification
Low specific gravity, softness, prismatic habit, solubility in water, and association help identify borax. Borax can, however, be confused with other borate minerals.

Figure 14.396 shows prismatic borax crystals, and Figure 14.397 shows scalenohedral crystals. Both specimens come from the same place in California’s Mojave Desert.

Physical Properties

hardness	2 to 2.5
specific gravity	1.7 to 1.9
cleavage/fracture	perfect {100}, good {110}/conchoidal
luster/transparency	vitreous, resinous/translucent
color	white, gray, less commonly light blue or green
streak	white

Properties in Thin Section
Borax is colorless in thin section, has three distinct cleavages, displays second-order interference colors, and shows anomalous interference colors in some orientations. Biaxial (-), a = 1.447 , β = 1.469, γ = 1.472, δ = 0.025, 2V = 40°.

Crystallography
Borax is monoclinic, a = 11.84, b = 10.63, c = 12.32, β = 106.58°, Z = 4; space group \(C\dfrac{2}{c}\); point group \(\dfrac{2}{m}\).

Habit
Euhedral borax crystals are most often stubby prisms with complex combinations of terminating faces. Borax is common in massive or granular aggregates.

Structure and Composition
Although the Na:B ratio is fixed, the amounts of (OH)^– and H₂O in borax are variable. The structure consists of chains of Na(H₂O)₆ octahedra connected to isolated groups of boron tetrahedra and double boron triangles. Weak van der Waals and hydrogen bonds link the octahedral chains to the boron groups, resulting in perfect prismatic cleavage.

Occurrence and Associations
Borax, associated with evaporite deposits in volcanic terranes, is the most common of the hydrous borate minerals. It is found in thick beds, similar to other salts, and as crusts and surface coatings. Common associated minerals are halite, NaCl; colemanite, CaB₃O₄(OH)₃•H₂O; ulexite, NaCaB₅O₆(OH)₆•5H₂O; and gypsum, CaSO₄•2H₂O.

Related Minerals
Borax dehydrates easily to tincalconite, Na₂B₄O₅(OH)₄•3H₂O.

Kernite Na₂B₄O₆(OH)₂•3H O

Origin of Name
Named after its only major occurrence, in Kern County, California.

Figure 14.398: Kernite from Boron, California; the specimen is 3.7 cm across

Figure 14.399: Kernite cleavage fragment, 8.8 cm long

Hand Specimen Identification
Kernite commonly resembles borax and some other borate minerals (Figure 14.398). Unlike other borates, however, it forms long sometimes splintery fragments when cleaved (Figure 14.399).

Physical Properties

hardness	3
specific gravity	1.90
cleavage/fracture	prismatic, perfect (100) and (001), poor (010)/uneven
luster/transparency	vitreous, pearly/transparent
color	colorless, white
streak	white

Properties in Thin Section
Kernite is colorless in thin section and has negative relief and two perfect cleavages. Interference colors range up to second-order red or orange. Borax has a moderate 2V and lower birefringence; ulexite and colemanite have only one cleavage and are biaxial (+). Biaxial (-), a = 1.454 , β = 1.472, γ = 1.488, δ = 0.034, 2V = 80°.

Crystallography
Kernite is monoclinic, a = 15.68, b = 9.09, c = 7.02, β = 108.87°, Z = 4; space group \(P\dfrac{2}{a}\); point group \(\dfrac{2}{m}\).

Habit
Kernite typically is in massive or coarse aggregates that typically cleave into long fragments.

Structure and Composition
Kernite‘s structure is complex, consisting of mixed chains of (BO₄)^5- tetrahedra and (BO)^3- triangles. The chains are linked by bonds to Na⁺ ions.

Occurrence and Associations
The only major occurrences of kernite are in Kern County, California, where it occurs with borax and ulexite.

Related Minerals
Kernite is similar to other borates in composition but, being identified by its distinctive cleavage, is rarely misidentified.

Ulexite NaCaB₅O₆(OH)₆•5H₂O

Origin of Name
Named for German chemist G. L. Ulex (1811–1883), who discovered it.

Figure 14.400: Acicular ulexite from Boron, California, 9 cm across

Figure 14.401: Clear ulexite with minor brown calcite, from Boron, California, 6.9 cm tall

Figure 14.402: “Television rock,” a variety of fibrous ulexite with fiber-optics properties

Hand Specimen Identification
Ulexite is similar to, and may be confused with, other borates. This mineral commonly forms soft rounded masses with a loose, “cotton ball” appearance (Figure 14.400). Less commonly it develops as prismatic, acicular or fibrous aggregates (Figure 14.401). The parallel fibrous nature of some specimens means that ulexite may transmit images like fiber optics (Figure 14.402).

Physical Properties

hardness	1 to 2.5
specific gravity	1.96
cleavage/fracture	perfect but rarely seen {010}/uneven
luster/transparency	silky/transparent to translucent
color	white
streak	white

Properties in Thin Section
Ulexite is biaxial (+), a = 1.491 , β = 1.505, γ = 1.520, δ = 0.029, 2V = 73°.

Crystallography
Ulexite is triclinic, a = 8.73, b = 12.75, c = 6.70, α = 90.27°, β = 109.13°, γ = 105.12°, Z = 2; space group \(P\overline{1}\); point group \(\overline{1}\).

Habit
Acicular and fibrous crystal aggregates are typical for ulexite. Crystals may form rounded masses with a delicate fuzzy “cotton ball” appearance. A variety called television rock contains massive, closely packed fibers, resulting in fiberoptic-like properties (Figure 14.402).

Structure and Composition
Ulexite‘s structure is complex, consisting of large B(O,OH)₃ and B(O,OH)₄ anionic groups, Ca²⁺ in 8- to 10-fold coordination, and Na⁺ in 6-fold coordination.

Occurrence and Associations
Similar to other borate minerals, ulexite forms in arid regions from evaporating water. It is commonly associated with borax, kernite, and colemanite.

Colemanite CaB₃O₄(OH)₃•H₂O

Origin of Name
Named after W. T. Coleman (1824–1893), the Californian who founded the California borax industry.

Figure 14.403: Colemanite from Boron, California, 1.3 cm across

Hand Specimen Identification
Colemanite is similar in chemistry and properties to other borates and distinguishing it from the other borates can be difficult. Excellent cleavage in one direction, color, transparency, and association help identify it. Figure 14.403 shows an example of colemanite from California’s Mojave Desert.

Physical Properties

hardness	4 to 4.5
specific gravity	2.42
cleavage/fracture	one perfect (010)/subconchoidal
luster/transparency	vitreous/transparent to translucent
color	colorless, white, gray
streak	white

Properties in Thin Section
Colemanite is colorless in thin section, appearing similar to other borates but having a higher index of refraction. Biaxial (+), a = 1.586 , β = 1.592, γ = 1.614, δ = 0.028,2V = 56°.

Crystallography
Colemanite is monoclinic, a = 8.74, b = 11.26, c = 6.10, β = 110.12°,Z = 4; space group \(P\dfrac{2_1}{a}\); point group \(\dfrac{2}{m}\).

Habit
Colemanite crystals vary, usually being short and prismatic but sometimes massive or granular.

Structure and Composition
Colemanite‘s structure consists of uneven sheets containing rings of (BO₄)^5- tetrahedra and (BO₃)^3-triangles.

Occurrence and Associations
Usually associated with ulexite, kernite, and borax, colemanite deposits form thick layers in ancient lake beds.