Experimental constraints on illite crystal morphology

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 f ormation. The experiments presented indicate that the occurrence of lath-sh aped mica-illite is due to a slow growth rate from supersaturated solutions . This lath-shaped crystallite is metastable as the ultimate stable shape i s hexagonal. A longer period of crystal growth will replace the mica-illite laths with crystallites of a hexagonal form. Thus the presence of lath-sha ped mica-illite indicates the existence of a metastable crystal form for th is mineral.