Smectite is the name for a group of sodium, calcium, magnesium, iron, lithium aluminun silicates which include the minerals sodium montmorillorite, calcium montmorillonite, saponite, nontronite, and hectorite. The rock in which these smectite minerals are usually dominant is bentonite.
The theoretical formula without considering lattice substitutions is (OH4Al4Si8O20 · N H2O (interlayer), and the theoretical composition without the interlayer material is 66.7% SiO2, 28.3% Al2O3, and 5% H2O. Smectite always differs from the theoretical formula because of substitutions within the lattice. Common substitutions are aluminum for silicon in the silica tetrahedral sheet which appears to be limited to less than 15 percent, and magnesium and iron for aluminum in the octahedral sheet. Substitutions in the octahedral sheet range from a small percentage to almost complete. Nearly total replacement of aluminum by magnesium results in the mineral saponite and by iron yields the mineral nontromite.
The mean chemical analysis taken from averaging 101 samples (Weaver and Pollard, 1973) gives the following percentages: SiO2 – 59.49, Al2O3 – 21.93, Fe2O3 – 3.77, FeO – 0.20, MgO – 3.55, CaO – 1.18, Na2O – 0.82, K2O – 0.34, TiO2 – 0.25, and H2O+ – 8.38. Typical analyses of sodium montmorillonite, calcium montmorillonite, saponite, nontronite, and hectorite are shown in Table 1.
The smectite minerals are very fine in particle size usually of the order of less than one micron. Most smectites are dry processed for use as drilling muds, foundry bond clays, ore pelletizing, and sealants. However, some are wet processed to produce very pure and uniform products for use as suspending agents, in cosmetics, special ceramics, pencil leads, detergents, catalysts, and other specialty applications. Fine particle size relatively pure smectites are excellent plasticizers and bonding agents even in very small percentage additions. Montmorillonite (Grimshaw, 1971) has a cross-breaking strength ranging from 1500 to 2500 gms per sq. cm. which is the highest of the clay materials.
One of the important properties of smectite is its high exchange capacity particularly for cations. Thus special clays can be produced with a particular cation on the exchange position. The range of cation exchange milliequivalents per 100 grams for sodium montmorillonite is 75 to 150.
Smectites lose their hydroxyl water in the range of 400 to 700°C and at temperatures of about 900°C several phases form including cristobalite, cordierite, mullite, spinel, enstatite, and anorthite depending upon the composition of the smectite. Smectites fuse at temperatures between 1200°C and 1400°C again depending on their composition.
The largest and highest quality sodium bentonite deposits in the world are located in south Dakota, Wyoming, and Montana. The major non-clay minerals in this sodium montmorillonite are quartz, cristobalite, feldspar, mica, and some zeolite. Most all of these non-clay minerals can be separated out by wet processing methods to provide an essentially pure sodium montmorillonite.
Bentonites in which calcium montmorillonites are the major mineral constituent are found in Arizona, Texas, and Mississippi. The non-clay minerals are essentially the same as those in the sodium montmorillonite or western bentonite. These calcium montmorillonites can be purified by wet process beneficiation methods.