Smectite-illite transformation

The values used for the wt% conversion term in the above equation were estimated from the calculated curves of Surdam and Crossey (1985). The redox calculation was made for acetic and malonic acids, assuming a conversion efficiency for malonic acid equivalent to that reported for the production of oxalic acid. This calculation was applied only to source rocks undergoing the smectite-illite transformation. The potential volume of plagioclase dissolved was calculated using the relationship ... 

The transformation of smectite to mixed layer smectite-illite, and ultimately to illite, with increasing temperature is an extremely important reaction in many sedimentary basins, including the northern Gulf of Mexico Basin (Hower et al., 1976 Boles and Franks, 1979 Kharaka and Thordsen, 1992). The water and solutes released and consumed by this transformation are major factors in the hydrogeochemistry of these basins, because of the enormous quantities of clays involved. Several reactions conserving aluminum or maintaining a constant volume have been proposed for this transformation (Hower et al., 1976 Boles and Franks, 1979). The reaction proposed below (Equation (4)) conserves aluminum and magnesium, and is probably a closer approximation based on the composition of formation waters in these systems ... 

Velde B. and Vasseur G. (1992) Estimation of the diagenetic smectite to illite transformation in time—temperature space. Am. Mineral. 77, 967-976. 

Colten-Bradley, V.A., 1987. Role of pressure in smectite dehydration - Effects on geopressure and smectite-to-illite transformation. The American Association of Petroleum Geologists Bulletin, Vol 71, no.ll, pp. 1414-1427... 

Illite clays may result from the weathering of micas and feldspars. Their formation in soils and sediments is favored by high K" " and moderate silica concentrations. When smectites or mixed-layer smectite/illite clays are buried in deep sedimentary basins, they are gradually transformed into more stable illites by a combination of time and temperature (diagenesis) (cf. Velde and Vasseur 1992 Huang et al. 1993 Cuadros and Linares 1996). The reaction involved might be... 

Elhott WC, Matisoff G (1996) Evolrration of kinetic models for the smectite to illite transformation. Clays Clay Minerals 44 77-87... 

Lanson B, Besson G(1992) Characterization of the end of smectite-to-illite transformation Decomposition of X-ray patterns. Clays Clay Minerals 40 40-52... 

Smectite group minerals, when present in the petroleum reservoirs, play an important role in the migration of hydrocarbons. At the shallow level of reservoir rocks smectite can exist, but at deeper level the increase of temperatiue transforms it to other minerals. Generally dioctahedral smectites are transformed to illite and trioctahedral smectites are transformed to chlorite, releasing the interlayer water molecules in both cases. That released water increases the pore fluid pressure that may lead to migration of the hydrocarbons. 

Fig. 7 shows the progressive transformation of montmorillonite to iliite/smectite interlayers by the gradual development of both the characteristic Cs and Rb high selectivity profiles observed for pure illite and the high Cs-Rb selectivity at+ race fadings. The data can be simulated (see table VI for the Ca - Cs case) using a consistent set of intrinsic selectivity coefficients and identical site group capacities for the Ca-Cs and... 

The experiments also indicate that WD may be an important mechanism for producing I/S at low temperatures in nature by a transformation mechanism (56). The percentage of illite layers formed by this mechanism is proportional to the number of WD cycles, and to the layer charge of the original smectite. Simple K-exchange does not produce stable illite layers in smectite therefore, these layers probably form by WD prior to deposition in subaqueous environments. The exception is found in high pH environments where illite layers may form without WD by chemical reaction, as has been reported previously for alkaline lakes (64, 65). 

The most widespread of the secondary minerals formed during the development of a laterite weathering profile are iron and aluminium ses-quioxides (Table 3.1). These may form either directly from the alteration of primary minerals, or else via a series of pathways involving the formation of intermediary sheet silicate minerals and clays (e.g. chlorite, illite, smectite, vermiculite and halloysite), which are then themselves broken down, stripped of their mobile ions and silica, and eventually converted to alumina and ferric oxyhydroxide residua (Figure 3.9). It is not possible to describe these mineral transformations in detail, but the key issue is that under tropical-type weathering conditions these transformation pathways lead to... 

Drits V A (1975) The stmctural and crystallochemical features of layer silicates. In Ciystallochemistiy of Minerals, and Geological Problems. AG Kossovskaya (ed), Nauka, Novosibirsk, p 35-51 (in Russian) Drits VA, Besson G, Muller F (1995) An improved model for stmctural transformations of heat-treated aluminous dioctahedral 2 1 layer silicates. Clays Clay Minerals 43 718-731 Drits VA, Lindgreen H, Saly n AL (1997) Determination of the content and distribution of fixed anunonium in illite-smectite by X-ray diffraction Apphcationto North Sea ilhte-smectite. Am Mineral 82 79-87 Drits VA, Lindgreen H, Salyn AL, Ylagan R, McCarty DK (1998) Semiquantitative determination of trans-vacant and cis-vacant 2 1 layers in ilhtes and ilhte-smectites by thermal analysis and X-ray diffraction. Am Mineral 83 1188-1198... 

Lindgreen H, Drits VA, Sakharov BA, Salyn AL, Dainyak LG (2000) lllite-smectite structural changes during metamorphism in black Cambrian Alum shales from the Baltic area. Am Mineral 85 1223-1238 Lindgreen H, Jacobsen H, Jacobsen HJ (1991) Diagenetic structural transformations in North Sea Jurassic illite/smectite. Clays Clay Minerals 39 54-69... 

Mineral transformations during diagenesis (e.g., illite smectite transition), metamorphism (e.g., recrystallization of clay minerals to biotites, amphiboles, etc) or alteration (e.g., serpentinization of mafic minerals) are likely to release the radiogenic noble gases that were produced within their lattices. This assumes that ... 

Fig. 4.1. Transformation of mixed-layer days of illite-smectite types (unordered) into ordered phase of vermiculite-chlorite type vs. transition of Fe into Fe. Paleozoic sediments of the Illizi Basin...

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