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14.8: Borate Minerals

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    18679
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    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 Al6-7BSi3O15(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.”

    14.396.jpg
    Figure 14.396: Borax from Boron, California; the sample is 30 cm across
    14.397.png
    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.

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

    14.400.png
    Figure 14.400: Acicular ulexite from Boron, California, 9 cm across
    14.401.png
    Figure 14.401: Clear ulexite with minor brown calcite, from Boron, California, 6.9 cm tall
    14.402.png
    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.

    14.403.png
    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.


    This page titled 14.8: Borate Minerals is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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