Aluminate

In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic anion with a central aluminium atom.

Aluminate oxyanions

Aluminium oxide (alumina) is amphoteric: it dissolves in both bases and acids. When dissolved in bases it forms hydroxyaluminate ions in the same way as aluminium hydroxide or aluminium salts. The hydroxyaluminate or hydrated aluminate can be precipitated and then calcined to produce anhydrous aluminates. Aluminates are often formulated as a combination of basic oxide and aluminium oxide, for example the formula of anhydrous sodium aluminate NaAlO₂ would be shown as Na₂O·Al₂O₃.

A number of aluminate oxyanions are known:

• The simplest is the approximately tetrahedral found in the compound Na₅AlO₄,
• framework ions in anhydrous sodium aluminate NaAlO₂ and monocalcium aluminate, CaAl₂O₄ made up of corner-sharing {AlO₄} tetrahedra.
• A ring anion, the cyclic anion, found in tricalcium aluminate, Ca₃Al₂O₆, which can be considered to consist of 6 corner sharing {AlO₄} tetrahedra.
• A number of infinite chain anions in the compounds Na₇Al₃O₈ which contains rings linked to form chains, Na₇Al₁₃O₁₀ and Na₁₇Al₅O₁₆ which contain discrete chain anions.

Mixed oxides containing aluminium

There are many mixed oxides containing aluminium where there are no discrete or polymeric aluminate ions. The spinels with a generic formula that contain aluminium as Al³⁺, such as the mineral spinel itself, MgAl₂O₄ are mixed oxides with cubic close packed O atoms and aluminium Al³⁺ in octahedral positions.

BeAl₂O₄, chrysoberyl, isomorphous with olivine, has hexagonal close-packed oxygen atoms with aluminium in octahedral positions and beryllium in tetrahedral positions.

Some oxides with the general formula of MAlO₃ sometimes called aluminates or orthoaluminates such as YAlO₃, Yttrium ortho-aluminate are mixed oxides and have the perovskite structure.

Some oxides such as Y₃Al₅O₁₂, usually called YAG, have the garnet structure.

Some materials that are known to form glass in binary combination with aluminium oxide are: rare earth oxides, alkaline earth oxides (CaO, SrO, BaO), lead oxide, and silicon dioxide.

Also, the Al₂O₃ (aluminate) system has been discovered to form sapphire-like glass ceramics. Often, this ability is based upon compositions in which interplay between glass forming ability and glass stability is approximately balanced.

Applications of aluminates

Sodium aluminate, NaAlO₂, is used industrially in dyeing to form a mordant and the hydrated forms are used in water purification, sizing of paper and in the manufacture of zeolites, ceramics and catalysts in the petrochemical industry. In the isomerization process of alkenes and amines Calcium aluminates are important ingredients of cements.

Aluminate suffix used in the naming of inorganic compounds

Examples are:

• Tetrachloroaluminate ion found in for example lithium tetrachloroaluminate.

Notes