Trioctahedral silicates

Ternary cation exchange, 216 Tetrahedral coordination, 102 Titration curves, 27-29, 154-159 Equivalence points, 28-29 pH-bufifering, 86-88 Acid clays and soils, 154-160 Total dissolved solids (TDS), 479, 491 Toxicity, 484 Indicators, 484 Ceriodaphnia, 484 Water fleas, 484 TIEs, 484 Metals, 484 Hard- metals, 12 Soft-metals, 12 Triazines, 345, 357 Trioctahedral silicates, 121... 

The basic mineral of the 2 1 trioctahedral silicates is talc with formula Mg3Si40io(OH)2. It is obvious that for this silicate Fe " will be the most abundant valence of iron and consequently the spectra consist predominantly of ferrous doublets [246, 248]. Similarly, this is also the basic spectral appearance for all... 

Experiments at pressures of one kilobar and above indicate that talc is produced between 100-300°C (Hohling, 1958). However, its occurrence in salt deposits (Braitsch, 1971 Fuchtbauer and Goldschmidt, 1959) and carbonates (Millot, 1964) indicate that it continues to be stable at lower temperatures. The experimental work of Siffert (1962) indicates that talc could be precipitated from concentrated basic solutions (pH > 9) where other magnesian silicates such as sepiolite and trioctahedral montmoril-lonite are not stable. 

The second facies is marked by the instability of the fully expanding dioctahedral phases and the existence of a kaolinite-illite tie-line (Figure 48b). In this facies the siliceous alkali zeolites (other than analcite) become unstable, the compositional range of the trioctahedral expanding phases is reduced and aluminous 14 8 chlorite-"allevardite"... 

The 1 1 clay-mineral type consists of one tetrahedral sheet and one octahedral sheet. These two sheets are approximately 7 A thick. This two-sheet type is divided into kaolinite (dioctahedral) and serpentine (trioctahedral) groups. The kaolinite minerals are all pure hydrous aluminum silicates. The different members are characterized by the manner of stacking of the basic 7 A layers (Brindley, 1961b). 

The two layer silicates are divided into the kaolinite (dioctahedral) and serpentine (trioctahedral) subgroups. The dioctahedral minerals are hydrous aluminum silicates containing minor amounts of other constituents. The trioctahedral minerals vary widely in composition and isomorphous substitution is common however, these minerals are relatively rare and chemical data are limited. 

Robbins, D. W. Strens, R. G. J. (1972) Charge transfer ferromagnesian silicates The polarized electronic spectra of trioctahedral micas. Mineral Mag., 38 551-63. 

Aqua regia extraction is a strong partial extraction method that dissolves carbonates, most sulphide minerals, some silicates like olivine and trioctahedral micas, clay minerals and primary and secondary salts and hydroxides (Salminen, 1995). It can be considered a quasi-total extraction method, since actual total concentrations can be higher. On the other hand, this leaching method overestimates the bioavailable amount of toxic elements in a soil since metals trapped in the silicate lattice are released very slowly in the environment and are not easily involved in plant nutrition processes. 

Figure 6. An example of a setting ideal for silicate precipitation in these caves. Modeled after a setting in Hell Below Cave where trioctahedral smectite formed with huntite moomnilk.

Silicates such as amorphous silica, trioctahedral smectite, and quartz, form in carbonate speleothems such as stalagmites, moonmilks, and crusts in the Guadalupe Mountains, New Mexico. In areas of relatively rapid carbonate mineral precipitation, amorphous silica was the common phase. In contrast, trioctahedral smectite and quartz formed where carbonate mineral precipitation was very slow and depositional conditions remained nearly constant for long periods of time. Other Mg-silicates such as kerolite formed in depositional environments between these two extremes. 

The structural distortions of phyllosilicate minerals have been investigated by solid-state Si nuclear magnetic resonance spectroscopy (NMR) [24], In trioctahedral layer silicates such as talc, the wSi chemical-shift value is related to the amount of >VA1 substitution and/or distortion of the tetrahedral sheet structure. 

Many of the layer silicate clays common in soils are based on the mica structure (shown in Figure 2.9b) in which two tetrahedral sheets sandwich a single sheet of octahedrally coordinated cations. Consequently, they are termed 2 1 layer sihcates. Conceptually, it is useful to start with the neutral framework of the talc and pyro-phyllite structures, representing the trioctahedral (Mg in the octahedral sheet) and dioctahedral (AF in the octahedral sheet) members of the 2 1 group. These have the ideal formulae given below ... 

In general, weathering conditions tend to favor the gradual conversion of trioctahedral layer silicate clays (with dominantly Mg " and Fe " in octahedral sites) to dioctahedral layer silicates (with dominantly AP and Fe in octahedral sites). Layer charge reduction accompanies this process. Expected weathering sequences of the two common micas, biotite and muscovite, in leaching environments are ... 

There is not much resistance to weathering in these minerals because of the relative lack of Si—O—Si bonding, especially in island silicates such as olivine. Layer silicate minerals rich in Mg (e.g., trioctahedral smectites, chlorite, serpentine) may form from the siliceous residue if leaching does not deplete in the weathering zone. 

Artificial synthesis of smectites, especially those with Mg in the octahedral sheet, is relatively easy. A solution saturated with monomeric silica, to which an equal number of moles of MgCl has been added, is adjusted to pH 11 with NaOH. This yields a crystalline precipitate of trioctahedral smectite at room temperature. Magnesium silicate with the 1 1 layer structure (serpentine) results if the Si/Mg mole... 

The laboratory synthesis at 2(fC of trioctahedral (Mg-bearing) layer silicate clays from solution has been found to be much easier than that of dioctahedral clays (e.g., kaolinite, montmorillonite). Provide a mechanism-based explanation for this, given that Al(OH)j is least soluble in the pH range of 5-9, while Mg(OH)2 is least soluble above pH 10. 

The 2 1 layer silicate minerals are sometimes defined on the basis of the number of octahedral positions occupied by cations. When two-thirds of the octahedral positions are occupied, such as in pyrophyllite (Al2Si40io(OH)2), the mineral is call dioctahedral, when all three positions are occupied, such as in talc (MgjSL OioCOH ), the mineral is called trioctahedral. This difference in composition of layer silicates can be fairly easily determined by x-ray diffraction, because each substitution slightly changes the dimensions of the unit cell. 

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