All Minerals

Illite

Illite is a mica-like clay mineral that carries its own potassium flux, making illitic natural clays behave very differently from kaolin even when their chemical analyses appear superficially similar.

Illite is one of the most important clay minerals in natural sedimentary clays, yet potters hear far less about it than kaolinite or bentonite. Many common clays, especially red-burning ones, contain significant illite. It is a mica-like clay mineral, typically containing substantial K₂O along with Al₂O₃ and SiO₂, and often some MgO and Fe₂O₃.

Structurally, illite is a 2:1 layer silicate. Each layer has an alumina-rich octahedral sheet sandwiched between two silica-rich tetrahedral sheets. Potassium ions occupy spaces between the layers and hold them together. This is quite different from kaolinite, whose 1:1 layers contain no interlayer potassium. Illite therefore has a chemistry and firing behavior that is much more fluxed than a relatively pure kaolin.

Natural illites vary considerably, so when it is mentioned in the mineralogy of a commercial clay, it generally describes a family of related minerals rather than one perfectly defined compound. Thus, much of the K₂O shown by a chemical analysis of an illitic clay is part of the clay mineral itself, not as feldspar. On firing, breakdown of the illite structure makes this potassium available as a flux. That is one reason many illitic clays begin densifying and vitrifying at much lower temperatures than kaolin-rich clays.

Illitic clays are commonly associated with fine particle size, good plasticity, iron-bearing minerals and substantial quartz. That being said, plasticity also depends on the percentage of illite, particle size and shape, mixed-layer minerals and the amount of non-plastic quartz and silt present. An illite-rich clay therefore typically matures early. Iron compounds commonly associated with it produce the familiar red, brown and buff colors of many natural clays.

For potters, perhaps the most useful way to think about illite is this: it is a naturally flux-bearing clay mineral. Illitic clays of lower iron content can be valuable in structural products since they require less energy to fire.

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