Clay minerals dehydration

Dolomite cement components were all sourced from outside the sand body, most probably from local or basinal mudrocks. Stable isotope data indicate a mixed organogenic-marine carbonate source, and precipitation at relatively low temperatures (s70°C, if pore fluids were sourced from clay mineral dehydration reactions during deep burial of Carboniferous mudrocks in the Rathlin basin 55°C if they were locally sourced). Thermobaric mass transfer was enhanced by tectonic pulsing and dolomite precipitation was driven by CO2 degassing. 

J. w. Geus, The interlayer collapse during dehydration of synthetic Na -beidellite a %a solid -state magic-angle spinning NMR study. Clays Clay Minerals. 25 457 (1992). 

Gonzales and Laird145 have shown that smectites abiotically catalyze dehydration of glucose to form furfural under conditions similar to those found in soils. Four smectite clay minerals were used (saturated with Na, Ca, Fe, or Al), and the formation of HMF and furfural was detected by high-pressure liquid chromatography. The polymerization of furfural may thus be a pathway to the formation of new humic materials in soils. 

Table III summarizes the parameters that affect Brrfnsted acid-catalyzed surface reactions. The range of reaction conditions investigated varies widely, from extreme dehydration at high temperatures in studies on the use of clay minerals as industrial catalysts, to fully saturated at ambient temperatures. Table IV lists reactions that have been shown or suggested to be promoted by Br nsted acidity of clay mineral surfaces along with representative examples. Studies have been concerned with the hydrolysis of organophosphate pesticides (70-72), triazines (73), or chemicals which specifically probe neutral, acid-, and base-catalyzed hydrolysis (74). Other reactions have been studied in the context of diagenesis or catagenesis of biological markers (22-24) or of chemical synthesis using clays as the catalysts (34, 36). Mechanistic interpretations of such reactions can be found in the comprehensive review by Solomon and Hawthorne (37).

Sepiolite clay (<100 mesh) was heated in air at 120°C in order to remove the zeolitic and surface bound water molecules. The partially dehydrated clay mineral was subsequently exposed to acetone vapor at room temperature for a period of four days. H and 29Si CP MAS-NMR experiments revealed that the acetone molecules penetrated into the microporous channels of the sepiolite structure. Broad line 2H NMR studies using acetone-d6 revealed that, in addition to fast methyl group rotations, the guest acetone-d6 molecules were also undergoing 2-fold re-orientations about the carbonyl bond. The presence of acetone-d6 molecules adsorbed on the exterior surfaces of the sepiolite crystals was also detected at room temperature. 

The reactions described and discussed here have been selected to extend the present analysis beyond surface processes and to include some consideration of certain chemical changes that also involve interactions within lattices of solids. The examples selected include reference both to surface catalytic properties and to dehydration reactions of clay minerals. 

Lin and Puls (2000) also observed that with aging of the clay minerals, the bonding strength between arsenic and clay mineral surfaces increases. This is probably because the surface-bonded arsenic slowly diffuses into internal micropores and the clay slowly dehydrates with time. Additionally, as the clay ages, the sorbed As(III) oxidizes to As(V), which exhibits a higher sorption affinity than As(III). 

Hashimoto, 1. and 3ackson, M.L., 1960. Rapid dissolution of allophane and Kaolinite-Halloysite after dehydration. Clay and Clay Minerals, 7th Conf., pp. 102-113. 

Walker, G.F., 1956. The mechanism of dehydration of Mg-vermiculite. Proc. Natl. Conf. Clays Clay Miner. 4th-Natl. Acad. ScL Natl. Res. Counc., Publ, 456 101-115. 

Figure 10.19. A. Relationship between the 8.45 T Cs MAS NMR room temperature chemical shifts of fully hydrated Cs-exchanged clay minerals and their degree of tetrahedral Al substitution. Open squares denote the dioctahedral minerals, open circles denote the trioctahedral minerals. Note that due to motional averaging in these samples, only one caesium resonance is observed. B. The same relationship for samples fully dehydrated at 450°C. The 2 lines correspond to the 2 Cs resonances observed in these samples. Note the similar behaviour of the dioctahedral and trioctahedral minerals when dehydrated. From Weiss et al. (1990a) by permission of the Mineralogical Society...

In contrast to the decompositions of many solids (including the hydrates discussed in the previous chapter), the dehydrations of hydroxides show some common patterns of behaviour in two broad groups the dehydroxylations of (i) simple hydroxides (Mg(OH)2, Ca(OH)2, etc.) and (ii) extended silicates (clays, minerals. 

Rowland, R.A., Weiss, E.J. and Bradley, W.F., 1956. Dehydration of monoionic montmorillonite. In A. Swineford (Editor), Proceedings Fourth National Conference Clays and Clay Minerals. Natl. Acad. Sci.—Natl. Res. Counc., Publ. No. 456, pp. 85—95. 

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