Smectite weathered

There are no unequivocal weathering reactions for the siUcate minerals. Depending on the nature of parent rocks and hydrauhc regimes, various secondary minerals like gibbsite, kaolinite, smectites, and iUites are formed as reaction products. Some important dissolution processes of siUcates are given, for example, by the following reactions (19). 

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

In terms of soil development and the development of soil horizons, the smectites and fine-grained micas are found in younger, less weathered soils. Kaolinite and amorphous clays are found in highly weathered soils. Considering a time sequence, at the beginning of formation, soil will contain more complex clays that weather to simpler forms over time. However, it is convenient to start with a description of the simpler layer silicate clays and then describe the more complex clays. 

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

The experiments indicate that WD can have a major effect on reactions that occur at the mineral-water interface. This process can rearrange clay 2 1 layers into more stable configurations, can substantially alter smectite selectivity for competing exchange cations, and can weather sparingly-soluble K-minerals in the presence of smectite. 

The palagonite is thermodynamically unstable and, hence, reacts with seawater to form various clay minerals, including smectites (montmorillonite, nontronite, and saponite), micas (celadonite), and zeolites (phillipsite). This chemical weathering involves uptake of Si, Al, Mg, Ca, Na, and K and the release of water, reversing to some extent, the elemental effect of palagonitization. These mineral alterations tend to proceed progressively from the outer margin of the pillow basalts to their interior. 

With the advent of stable isotope paleoaltimetry towards the turn of the millennium the stable isotope and tectonics communities have witnessed an increasing number of isotopic mineral proxies developed to address the long-term topographic histories of orogenic belts and continental plateaus. These proxies include calcite from paleosols (see for example Quade et al. 2007, this volume and references therein), fluvial and lacustrine rocks the phosphate and carbonate component of mammal teeth (Kohn and Dettman 2007, this volume and references therein), smectite and kaolinite from paleosols, weathered sediments and volcanic ashes (e.g., Chamberlain et al. 1999 Takeuchi and Larson 2005 Mulch et al. 2006a) as well as white mica from extensional shear zones and fluid inclusions in hydrothermal veins (e.g., Mulch et al. 

Chamberlain et al. (1999) measured the oxygen isotope composition of authigenic kaolinite, smectite and illite in weathered horizons from intermontane basins on the east side of the Southern Alps. Neither smectite nor illite produced useful results, as reflected in the... 

Gjems, O., 1963. A swelling dioctahedral clay mineral of a vermiculite-smectite type in the weathering horizons of podzols. Clay Miner., 5 183-193. 

As an example of these ideas, an activity-ratio diagram for control of Al(III) solubility by secondary minerals in an acidic soil solution will be constructed. The Jackson-Sherman weathering scenario14 indicates that when soil profiles are leached free of silica with fresh water, 2 1 layer-type clay minerals are replaced by 1 1 layer-type clay minerals, and ultimately these are replaced by metal oxyhydroxides. This sequence of clay mineral transformations can be represented by the successive dissolution reactions of smectite, kaolinite, and gibbsite ... 

A lot of work will have to be done before we can prove or reject this theory. The first problem is the precipitation of clay minerals directly from the solution. It is nearly impossible to simulate this process in the laboratory, because of the long time needed to initiate the precipitation from relatively diluted solutions. Mostly, clay minerals are formed by recristallization of other silicates which are less stable under weathering conditions. After proving that iron smectite indeed precipitates on the ocean floor, the quantity of this material will have to be determined in order to calculate whether a greater portion of iron can be removed from the seawater in this way. 

A recent contribution to mass-balance modeling of weathering (Bowser and Jones, 2002) utilizes a spreadsheet graphical method to interpret mass balance in watershed systems in place of the strictly numerical solution methods. Key to the approach is to solve the mass-balance equation for a 10 X 10 matrix as a function of mineral composition (specifically in terms of plagioclase feldspar and smectite compositions). Exploration by means of models that cover mineral compositional space limited the range of possible compositions and restricted the range of possible mass-balance solutions. Figure 1 (after Bowser and Jones, 2002) is an example of the spreadsheet approach applied to the Sierra Nevada ephemeral... 

Banfield J. F. and Barker W. W. (1994) Direct observation of reactant—product interfaces formed in natural weathering of exsolved, defective amphibole to smectite evidence for episodic, isovolumetric reactions involving structural inheritance. Geochim. Cosmochim. Acta 58, 1419-1429. 

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