Silicate Chemical Formulas

Silicate chemical formulas are written to communicate not just composition but also structural information. The convention is to list cations in order of decreasing coordination number — from the largest, most loosely bonded cations at the beginning to the most tightly bonded tetrahedral cations at the end. Reading the formula from left to right therefore traces a path from the lowest-density, highest-coordination sites to the highest-density, lowest-coordination sites of the crystal structure. ^[1]

The General Silicate Formula

The general formula for silicates is:

W_aX_bY_cO_d(Z_eO_f)A_g

Each letter represents a specific crystallographic site, defined by coordination number. The W cations are the largest, occupying highly irregular 10 to 12-fold coordination polyhedra — positions such as the A-site of amphiboles. The X cations sit in approximately 8-fold coordination; Ca in diopside and Na in edenite are examples. The Y cations occupy 6-fold octahedral coordination sites, filled by medium-sized cations such as Mg, Fe, Al, Mn, and Ti. The Z position hosts the tetrahedral cations — primarily Si⁴⁺, with Al³⁺ as the most common substitute — in 4-fold coordination. The A position at the end of the formula is reserved for large anions such as OH⁻, F⁻, and Cl⁻ in hydrous silicates such as the micas, amphiboles, and chlorites. ^[1]

Reading the Z:O Ratio

The ratio of Z-site cations to total oxygen atoms within the formula directly reveals the silicate structural class, because the degree of tetrahedral polymerization is encoded in this ratio. An isolated orthosilicate such as fayalite (Fe₂SiO₄) has a Z:O ratio of 1:4 (one Si per four O). A framework silicate such as quartz (SiO₂) has a Z:O ratio of 1:2, reflecting the fact that every oxygen is shared between two tetrahedra. All intermediate classes fall between these extremes (see the Silicate Minerals page for the complete table).

In disilicates and orthosilicates, the formula often contains more oxygen atoms than are needed to account for the silicon/aluminum tetrahedra alone. This extra oxygen may be listed separately after the Y position or included in the A position at the end of the formula. ^[1]

Worked Examples (Table 11.3)

The table below illustrates how the general formula applies to six common silicate minerals, showing the assignment of each cation to its W, X, Y, or Z site.

Note that these conventions are followed as consistently as possible, but the reader should be aware that historical or other reasons have led some published silicate formulas to deviate from this ordering scheme. ^[1]

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