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Mineral group · Silicate

Illite

K₀.₆₅Al₂(Al₀.₆₅Si₃.₃₅)O₁₀(OH)₂

The dominant clay mica in shales and soils.

Hardness1–2
Crystal systemMonoclinic
ClassSilicate
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Illite specimen

Illite is a silicate that is formed by the weathering or hydrothermal alteration of muscovite-phengite and also authigenic alteration of K-spar or recrystalliza — Photo: Shannon Heinle · CC0

Illite

The commonest non-expanding clay mineral — 'hydromica' — a potassium-bearing phyllosilicate of soils and shales.

Intro adapted from Wikipedia (CC BY-SA).

Illite is a mineral group, with the chemical formula K₀.₆₅Al₂(Al₀.₆₅Si₃.₃₅)O₁₀(OH)₂.

It crystallizes in the monoclinic crystal system.

Key properties: Mohs hardness 1–2; specific gravity 2.75; typically white or gray; refractive index 1.572; optically biaxial.

Collections: Everyday Minerals.

Index entry. This entry combines official IMA/RRUFF species data with published physical-property data and a Wikipedia introduction — a full written entry hasn’t been published yet. Browse more I minerals or help write this entry.