Illite, also called hydromica or hydromuscovite,[6] is a group of closely related non-expanding clay minerals.

[5] The space between this T-O-T sequence of layers is occupied by poorly hydrated potassium cations which are responsible for the absence of swelling.

Due to the small size, positive identification usually requires x-ray diffraction or SEM-EDS (automated mineralogy) analysis.

Illite occurs as an altered product of muscovite and feldspar in weathering and hydrothermal environments; it may be a component of sericite.

The iron-rich member of the illite group, glauconite, in sediments can be differentiated by x-ray analysis.