Aluminate
In chemistry 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 aluminum atom.
Aluminium oxide 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:
There are many mixed oxides containing aluminium where there are no discrete or polymeric aluminate ions. The spinels with a generic formula A2+
B3+
2O2−
4 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 YAl5O8, usually called YAG, have the garnet structure.
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