Observations of natural minerals and experimental run products show that I-S (illite-smectite mixed-layer minerals) with a high illite content as well as pure mica-illite crystallites can have a lath shape (aspect ratio >3). This shape is common to the I-S minerals under a variety of conditions of formation. The experiments presented indicate that the occurrence of lath-shaped mica-illite is due to a slow growth rate from supersaturated solutions. This lath-shaped crystallite is metastable as the ultimate stable shape is hexagonal. A longer period of crystal growth will replace the mica-illite laths with crystallites of a hexagonal form. Thus the presence of lath-shaped mica-illite indicates the existence of a metastable crystal form for this mineral.