Borate Mineral
borate mineral, any of various naturally occurring compounds of boron and oxygen. Most borate minerals are rare, but some form large deposits that are mined commercially.
alpha = 1.658–1.662
beta = 1.662–1.667
gamma = 1.668–1.673
alpha = 1.445
beta = 1.469
gamma = 1.472
alpha = 1.586
beta = 1.592
gamma = 1.614
alpha = 1.492–1.495
beta = 1.501–1.510
gamma = 1.516–1.520
alpha = 1.454
beta = 1.472
gamma = 1.488
alpha = 1.83–1.85
beta = 1.83–1.85
gamma = 1.97–2.02
alpha = 1.569–1.576
beta = 1.588–1.594
gamma = 1.590–1.597
alpha = 1.575–1.670
beta = 1.646–1.728
gamma = 1.650–1.732
omega = 1.461
epsilon = 1.474
alpha = 1.491–1.496
beta = 1.504–1.506
gamma = 1.519–1.520
name
colour
lustre
Mohs hardness
specific gravity
boracite
colourless or white
vitreous
7–7½
2.9–3.0
borax
colourless to white; grayish, bluish, greenish
vitreous to resinous
2–2½
1.7
colemanite
colourless; white, yellowish, gray
brilliant vitreous to adamantine
4½
2.4
inyoite
colourless, becoming white and cloudy after partial dehydration
vitreous
2
1.7
kernite
colourless
vitreous
2½
1.9
ludwigite
dark green to coal black
silky
5
3.6 (lud) to 4.7 (paig)
priceite
white
earthy
3–3½
2.4
sussexite
white to straw-yellow
silky to dull or earthy
3–3½
2.6 (szai) to 3.3 (suss)
tincalconite
white (natural); colourless (artificial)
vitreous
1.9
ulexite
colourless; white
vitreous; silky or satiny
2½
2.0
name
habit or form
fracture or cleavage
refractive indices
crystal system
boracite
isolated, embedded, cubelike crystals
conchoidal to uneven fracture
orthorhombic (isometric above 265 degrees C)
borax
short prismatic crystals
one perfect, one good cleavage
monoclinic
colemanite
short prismatic crystals; massive
one perfect, one distinct cleavage
monoclinic
inyoite
short prisms and coarse crystal aggregates; geodes; drusy crusts; granular massive
one good cleavage
monoclinic
kernite
very large crystals; fibrous, cleavable, irregular masses
two perfect cleavages
monoclinic
ludwigite
fibrous masses; rosettes; sheaflike aggregates
no observed cleavage
orthorhombic
priceite
soft and chalky to hard and tough nodules
earthy to conchoidal
triclinic(?)
sussexite
fibrous or felted masses or veinlets; nodules
probably orthorhombic
tincalconite
found in nature as a fine-grained powder; physical properties are given for artificial pseudocubic crystals
hackly fracture
hexagonal
ulexite
small nodular, rounded, or lenslike crystal aggregates; fibrous botryoidal crusts; rarely as single crystals
one perfect, one good cleavage
triclinic
Borate mineral structures incorporate either the BO3 triangle or BO4 tetrahedron in which oxygen or hydroxyl groups are located at the vertices of a triangle or at the corners of a tetrahedron with a central boron atom, respectively. Both types of units may occur in one structure. Vertices may share an oxygen atom to form extended boron–oxygen networks, or if bonded to another metal ion consist of a hydroxyl group. The size of the boron–oxygen complex in any one mineral generally decreases with an increase of the temperature and pressure at which the mineral forms.
Two geological settings are conducive for the formation of borate minerals. The first is commercially more valuable and consists of an environment where an impermeable basin received borate-bearing solutions that resulted from volcanic activity. Subsequent evaporation caused precipitation of hydrated alkali and alkaline-earth borate minerals. With increased depth of burial resulting from additional sedimentation, beds of compositionally stratified borates crystallized as a consequence of temperature and pressure gradients. Because evaporation must occur for precipitation of the borates, such basin deposits usually occur in desert regions, as for example the Kramer district of the Mojave Desert and Death Valley in California, where enormous beds of stratified kernite, borax, colemanite, and ulexite are recovered, primarily by stripping away the overburden and mining the borates by classical open-pit techniques. Other noteworthy evaporite deposits occur in the Inderborsky district of Kazakhstan and in Tuscany, Italy. The sequence of precipitating alkali borates can be duplicated in the laboratory because the temperatures and pressures of their formation are low and easily accessible. Solutions of the alkali borates and the addition of metal ions such as calcium and magnesium result in the precipitation of yet other borate compounds. Among the borates commonly found in evaporite deposits are borax, colemanite, inyoite, kernite, and tincalconite.
The second geologic setting for borate minerals is a metamorphic carbonate-rich environment, where they are formed as a result of alteration of the surrounding rocks by heat and pressure; similar borates also occur as nodules in some deeply buried sediments. These compounds were formed at relatively high temperatures and usually consist of densely packed BO3 triangles associated with such small metal ions as magnesium, manganese, aluminum, or iron. The origin of these borates is not as obvious as that of the evaporite varieties. Some were produced by the reaction of boron-bearing vapour derived from hot intruding granites during metamorphism; others are the recrystallization products of evaporite borates. Numerous borosilicates (e.g., dumortierite and tourmaline) were formed under these conditions. Compounds of this type contain both BO3 triangular units and SiO4 tetrahedral units. Among the borate minerals associated with metamorphosed environments are boracite, ludwigite, sussexite, and kotoite.
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