Investigations in the system K2O-MgO-Al2O3-SiO2-H2O were made in an attempt to synthesize micas similar to illite and to determine their upper stability limits. These are representative of materials referred to by many clay mineralogists as illite, hydromuscovite and K-bentonite. Gels of these compositions were treated hydrothermally at temperatures above 250°C and at pressures above 10,000 lb/in.2. Natural illites and related minerals were also treated under the same conditions.

It is possible to prepare phases having the properties of illite. This is achieved below 500°C but above this temperature well-crystallized micas plus other phases are obtained. The partially disordered micas formed from K-deficient compositions give the x-ray diffraction pattern of the 3T polytype, whereas the well-crystallized micas obtained with more K2O or at higher temperatures are the 1M polytype. Once formed at lower temperatures, however, the 3T mica is persistent and not readily transformed at higher temperatures. The 2M polytype, which is reported to be the stable form for pure muscovite at the temperatures of this investigation, was not obtained.

Compositions with the same alumina: silica ratio as muscovite but with less potassium do not yield a mica-type mineral as a single phase under the conditions of these experiments. However, those in which the lower potassium content is compensated by less substitution and lower layer charge do yield illite alone. These studies indicate that the differences in properties between illites and well-crystallized micas are a function of composition as well as of the temperature of formation.