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.

The oxidation potential of dithionite (Na2S2O4) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At the same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics occurs at approximately pH 7.3. A buffer is needed to hold the pH at the optimum level because 4 moles of OH are used up in reaction with each mole of Na2S2O4 oxidized. Tests show that NaHCO3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline goethite dissolved more slowly, but dissolved during the two or three 15 min treatments normally given for iron oxide removal from soils and clays.

A series of methods for the extraction of iron oxides from soils and clays was tested with soils high in free iron oxides and with nontronite and other iron-bearing clays. It was found that the bicarbonate-buffered Na2S2O4-citrate system was the most effective in removal of free iron oxides from latosolic soils, and the least destructive of iron silicate clays as indicated by least loss in cation exchange capacity after the iron oxide removal treatment. With soils the decrease was very little but with the very susceptible Woody district nontronite, the decrease was about 17 percent as contrasted to 35–80 percent with other methods.

A study was made of the changes in viscosity of Wyoming bentonite suspensions with variations in cation ionization, base saturation, clay concentration, sodium chloride concentration and type of exchangeable cation. There is little change in the viscosity with decreasing cation ionization until a certain threshold value is reached. Below this value the viscosity increases rapidly, indicating that the repulsive potential barrier has been reduced to the order of the kinetic energy of the particles. Unfortunately, a quantitative estimate of the ionization was not possible at this value because of the high salt content of the system.

Exchangeable aluminum was found to act as a strong bonding agent between clay particles in the pH range 4.50–5.50. This was explained on the basis of the formation of multivalent, aluminum-hydroxyl complex ions.

Swelling pressures calculated from diffuse double-layer theory previously had been experimentally verified for Na-montmorillonite at clay concentrations above 10 percent, but published data at clay concentrations less than 5 percent consistently showed osmotic pressures much lower than the maximum of about 40 cm water calculated from theory. Swelling pressures of Na-montmorillonite in 4–5 percent orientated suspensions were therefore measured in an specially constructed osmometer in various solutions of NaCl up to 1.25 mN. The osmometer construction allowed solutions of desired concentration to be flushed continually across the membrane separating solution from clay. It was found that soluble impurities present in the clay decreased swelling pressures to about 1 cm of water. When these were removed during sample preparation, most efficiently by ultrafiltration through a dialysis membrane, swelling pressures measured were in satisfactory agreement with calculated values. The impurities were not identified chemically, but buffer curves of the solution toward HCl were determined.

Glycerol-ethanol sodium salted clay pastes prepared from clays extracted directly from the soils and not allowed to dry were found to yield superior x-ray diffraction diagrams for the identification of the major clay minerals than pastes prepared from clays that suffered dehydration after extraction.

Prevention of drying of soil clays after their extraction from the soils is essential for proper identification of the clay minerals. This is particularly true for identification of hydrated halloysite and for differentiation between high exchange forms and low exchange forms of montmorillonite. This differentiation is based on the finding that glycerol-ethanol salted potassium-saturated montmorillonite pastes prepared from clay suspensions have a 001 spacing of 18.5 Å whenever the exchange capacity ranges between 92 and 95 meq/100 g oven-dry clay, and 14.5 Å whenever the exchange capacity ranges between 115 and 135 meq/100 g. Either both these spacings or a mixed-layer spacing appears in montmorillonites having exchange capacities between 95 and 115 meq/100 g.

Saturation of pure vermiculite minerals brings about contraction in the 001 spacing from 14.2 Å to 10.6 Å only after air drying in those forms with exchange capacities ranging between 200 and 207 meq/100 g water-free mineral, but in vermiculites with an exchange capacity of 250 meq/100 g this contraction occurs even without drying. Both of these forms of vermiculite are found in soils.

Several sets of “standardized” natural clay mineral mixtures are necessary for quantitative x-ray analysis of soil clays.