13.5: Oxygen and Other Common Elements

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Oxygen is the most abundant element in Earth’s crust, accounting for about 60 wt. %. It is not surprising, then, that O^2– is the most common anion in minerals. The ionic radius of oxygen varies from about 1.27 Å to 1.34 Å, depending on its coordination number.

The most abundant crustal cations include Si⁴⁺, Al³⁺, Fe²⁺, Fe³⁺, Mg²⁺, Mn²⁺, Na⁺, Ca²⁺, and K⁺. The table below lists the typical coordinations for each (with oxygen) and gives example minerals. A range of radius values is given for each cation because cation radius varies slightly with structure and coordination. The C.N.s in this table are, for the most part, consistent with Pauling’s first rule.

Cation size increases from the bottom of the table to the top, and coordination number increases too. Small cations such as C⁴⁺ and B³⁺ can have triangular coordination. Si⁴⁺ is nearly exclusively tetrahedral, while Al³⁺ may be either tetrahedral or octahedral. Although radius ratios predict only 6-fold coordination for Fe³⁺, in natural crystals Fe³⁺ can be either tetrahedral or octahedral. Other elements can be in octahedral coordination as well. The alkalis and the alkaline earths are the only elements that normally can be in cubic or dodecahedral coordination.

When contradictions between nature and radius ratios occur, it is usually for cations in highly irregularly shaped sites. For example, the aluminum in andalusite is in both 5-fold and 6-fold coordination, some magnesium in anthophyllite is in 7-fold coordination, and the potassium in microcline is in 10-fold coordination. We have not considered 5-fold and 7-fold coordination in this chapter because no regular polyhedra have five or seven vertices – and, consequently, application of Pauling’s first rule is problematic.

Sometimes we wish to show cation coordination in a mineral formula. Traditionally, this has been done using superscript Roman numerals. For example, we may write andalusite’s formula as Al^VAl^VISiO₅ to remind us of the unusual aluminum coordination. We may write magnetite’s as Fe^IVFe^VI₂O₄ to show that iron occupies two differently coordinated sites. Today, however, many crystallographers and chemists use Arabic numerals in square brackets instead of Roman numerals to show coordination numbers.