Aluminum is a strongly oxophilic element; that is, it combines strongly and preferentially with oxygen. The naturally occurring forms of Al are, consequently, oxides or oxo anion species. Many minerals are aluminosilicates: network structures in which aluminum and silicon are associated with oxygen

25Borax should be written Na2[B405(0H)4] ■ 8H2O, where the B4O5 unit can be regarded as a B4O4 ring, with the fifth O atom bridging two B atoms across the ring.

in insoluble, relatively inert materials. These are discussed in more detail in Chapter 12. Aluminum metal itself is highly reactive, but an impervious layer of oxide that forms upon exposure to air renders it inert. The chemical stability of these oxo compounds leads to the general assumption that Al could be considered a nontoxic element. However, it is now recognized that aluminum can be converted to soluble forms—for example, under acidic conditions such as those produced by acid rain—and that these soluble forms are toxic to plants, fish, and possibly to other animal life as well.

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