Borates, Natural

a class of minerals, salts of boric acids. They include orthoborates, metaborates, tetraborates, and salts of polyboric acids. The bases of the structure of natural borates are boric oxygen triangles [BO₃]^3– and tetrahedrons (BO₄)^5– with possible substitution of part of the O by OH^1– or combinations thereof. Analogously to silicic oxygen tetrahedrons of silicates, the boric oxygen triangles and tetrahedrons, when combining, may form chain, strip, stratified, or shell structures. There are anhydrous and hydrous borates, the latter being more common. Most borates are white or colorless; they are crystallized predominantly into rhombic and monocline systems.

Natural borate deposits arise in different ways. The largest of them (storing tens and hundreds of millions of tons of B₂O₃) are vulcanogenic-sedimentary deposits. Endogenetic deposits (storing up to tens of millions of tons of B₂O₃) are associated almost exclusively with skarns. Halogenic-sedimentary deposits (containing several millions of tons of B₂O₃) occur jointly with the salt deposits of dried-up marine basins.

The main industrial minerals from vulcanogenic-sedimentary deposits are sodium, calcium, and calcium-sodium hydrous borates:

ulexite NaCa [B₅O₆(OH)₆] · 5H₂O

kernite and borax Na₂[B₄O₆(OH)₂] · 3H₂O

and Na₂ [B₄O₅(OH)₄] · 8H₂O

colemanite Ca [B₃O₄(OH)₃] · H₂O

inyoite Ca [B₃O₃(OH)₅] · 4H₂O

priceite 5CaO · 6B₂O₃ · 9H₂O

In halogenic-sedimentary deposits, calcium, calcium-sodium, and magnesium hydrous borates predominate, including ulexite, colemanite, inyoite, inderite Mg[B₃O₃(OH)₅]5H₂O, szaibelyite Mg₂[B₂O₄(OH)] (OH), and hydroboracite CaMg[B₃O₄(OH)₃]₂3H₂O. Boracite is found in some deposits.

The main industrial minerals of skarn deposits are basically anhydrous magnesium and magnesium-ferrous borates: suanite Mg₂[B₂O₅], cotoite Mg₃[BO₃]₂, minerals of the lud-wigite group

(Mg, Fe²⁺)₂ (Fe³⁺, Al, Sn⁴⁺) [BO₃] O₂

and lower temperature szaibelyites, developing with substitution of other borates.

The most important deposits of the vulcanogenic-sedimentary type are found in the USA (California and neighboring states), and in Peru, Chile, Argentina, Turkey, and China (Tibet).

Endogenetic deposits are found in the Soviet Union (Western and Eastern Siberia, the Far East, Kazakhstan, and Middle Asia), China (northeastern provinces), North Korea, and Rumania.

Industrial deposits of halogenic-sedimentary type are known only in Kazakhstan (at Inderskoe and elsewhere).