Sepiolite, precipitation

In the calculation results (Fig. 24.1), amorphous silica, calcite (CaCCF), and sepiolite precipitate as water is removed from the system. The fluid s pH and ionic strength increase with evaporation as the water evolves toward an Na-C03 brine (Fig. 24.2). The concentrations of the components Na+, K+, Cl-, and SO4- rise monotonically (Fig. 24.2), since they are not consumed by mineral precipitation. The HCO3 and Si02(aq) concentrations increase sharply but less regularly, since they are taken up in forming the minerals. The components Ca++ and Mg++ are largely consumed by the precipitation of calcite and sepiolite. Their concentrations, after a small initial rise, decrease with evaporation. 

Fig. 24.1. Volumes of minerals (amorphous silica, calcite, and sepiolite) precipitated during a reaction model simulating at 25 °C the evaporation of Sierra Nevada spring water in equilibrium with atmospheric C02, plotted against the concentration factor. For example, a concentration factor of x 100 means that of the original 1 kg of water, 10 grams remain.

Calcite and sepiolite precipitate in large part because of the effects of the escaping C02. The corresponding reactions are,... 

Consequently, because the equilibrium value of the ion activity product is 10 8-35, the water is slightly undersaturated with respect to calcite, its Ca2+ content is fixed by C02 pressure and pH in other words, because the product of aCa2+ and aCo32 is a constant, aCa2+ and hence mca2+ can be expressed as a negative term on the right side of the electrical balance equation. After sepiolite precipitates, it can be handled similarly. The... 

Sepiolite precipitates from the silica-rich alkaline fluid according to the reaction ... 

Moisture. The presence of water in a filler is not usually beneficial. Most fillers added to adhesives have a moisture content lower than 1 wt%. Only precipitated silicas and sepiolite contain about 5-10 wt% moisture. For some applications, fillers must be completely dried to exhibit adequate performance. Moisture absorbed on the surface of fillers impacts the rate and extent of curing of rubber base adhesives. 

Figure 24.3 compares the calculated composition of the evaporated water, concentrated 100-fold and 1000-fold, with analyses of waters from six saline alkaline lakes (compiled by Garrels and Mackenzie, 1967). The field for the modeled water overlaps that for the analyzed waters, except that Ca++ and Mg++ are more depleted in the model than in the lake waters. This discrepancy might be explained if in nature the calcite and sepiolite begin to precipitate but remain supersaturated in the fluid. 

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. 

Sepiolite and palygorskite have a rather special composition and seem to be related to specific mineral parageneses. They appear to be stably associated with montmorillonite, corrensite, serpentine, chert, sulfates, carbonates and various salts. They are found in deposits typified by processes of chemical precipitation or solution-solid equilibria (Millot, 1964) and are therefore rarely associated in sediments with large quantities of detrital minerals. Their chemical environment of formation is in all evidence impoverished in alumina and divalent iron. Their frequent association with evaporites, carbonates and cherts indicate that they came from solutions with high chlorinity. 

SiO concentrations of the supernatant liquid. The result of these experiments is to establish that the precipitation of sepiolite from solution 2+... 

Thus the solution always contains Mg in approximately constant abundance, which makes it effectively a perfectly mobile component. The same is true for H+ since pH changes little after precipitation of the sepiolite even though the reaction consumes (OH). The experimental system is then "open" with respect to these two components. A determination of the solubility product constant of a natural iron-calcium-aluminous sepiolite confirms generally the above results (Christ, et al , 1973). 

At this point (the same pH as that of Wollast, t al., 1968) sepiolite begins to precipitate. In experiments maintaining pH at values above 9, montmorillonoids and talc were formed. Chemical analysis of the precipitates reveal a greater proportion of magnesium as the pH of the experiment is increased. Recalling the information on amorphous silica solubility, a two-fold increase in solubility of SiC>2 occurs between pH 8 and 10.5 (Krauskopf, 1959)—and thus at higher pH it could be expected that relatively less silicious phases would precipitate where the masses of Mg and Si are fixed. Final concentrations of Mg-Si in solution were not determined by Siffert and therefore thermodynamic calculations of mineral stabilities cannot be made. 

M = montmorillonites I illite Chi chlorite Pa = palygorskite Sep = sepiolite ppt = zone of silicate precipitation. 

The use of the "closed system" to describe the assemblages in these closed basins seems justified in that frequently, most always, in fact, the number of clay minerals present in the sediments discussed above is two or more. The omnipresence of amorphous silica or chert raises the total number of phases to three. In an essentially three-component system, Mg-Si-Al or possibly four if H+ is considered, this indicates that the chemical components of the minerals are present in relatively fixed quantities in the chemical system which produces the mineral assemblages. None of the first three components is "mobile", i.e., its activity is independent of its relative mass in the solids or crystals present. However, there are sediments which present a monophase assemblage where only one variable need be fixed. Under these conditions sepiolite can be precipitated from solution and pre-existing solid phases need not be involved. 

The relationship between aluminous sepiolite and the magnesian form is not at all clear as yet. The question of whether or not aluminous sepiolite can be precipitated directly from solution should be posed since alumina concentration in alkaline solutions (pH 7-9) is assumed to be quite low. For the present, palygorskite should probably be considered as a phase produced through solid-solution equilibria due to its high alumina content. Possibly most sepiolites are produced in this way. Even the inonomineralic deposits in saline lakes or deep sea cores contain sepiolites with high alumina contents (Parry and Reeves, 1968 McLean, et al., 1972). 

Buffering of pH during the early heavier precipitation of calcite and sepiolite is clear and is reflected in a near constancy of HCCV and CO32. However, after Ca2+ and Mg2+ are substantially reduced, the pH again rises with further concentration. Because the Sierra waters are so low in sulfate, gypsum does not precipitate abstraction of Ca2+ as calcite never permits the solubility product of gypsum to be exceeded. 

The experimental values for the free energies of formation of kaolinite and sepiolite are given in Tbble II. The value of -907.7 +1.33 kcal/mol recommended for kaolinite, is the mean of three recomputed free energies of formation weighed equally in the computation, and was obtained from calorimetry, dissolution, and precipitation data. Several values in the -905 to -906.0 kcal/mol range probably reflect the more soluble nature of small particles typically present in bulk samples. 

Similarly pure beds of kerolite and sepiolite are found in the modern and Pleistocene groundwater wetlands of Amboseli, Kenya (Stoessell and Hay, 1978 Hay and Stoessell, 1984 Hay et al, 1995). The concentration of dissolved silica is also important at higher ratios of Si02 to Mg, chain-structure clay (sepiolite-palygorskite) can precipitate directly from solution, as is also the case at Amargosa. 

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