Hydrothermal environments clay minerals

The geological environments which form clay minerals can be basically divided into five types weathering, sedimentation, burial, diagenetic and hydrothermal alteration. The weathering environment frequently presents a chemical system where T,P are constant and many chemical elements are mobile, usually they enter solution from the rocks present at the earth s surface through the process of hydrolysis. The major problems are (a) Determination of rates of reaction among the minerals present,... 

It is most likely then that the effective (although metastable) SiO equilibria in most geological environments of low temperature and pressure, weathering, sedimentation and the early stages of compaction as well as surface hydrothermal alterations, are governed by the solubility and precipitation of amorphous silica in aqueous solution. As a result, the existence of quartz in an assemblage of clay minerals in these environments does not necessarily represent a compositional limit or saturation with respect to SiC and, therefore, such an assemblage cannot be considered, a priori, as a system with silica as an effective component in excess. 

In the discussion thus far, the application of systems with completely mobile components has been restricted to bed-rock weathering, sedimentation and free-flowing aquifer environments. As a first approximation in other geological situations, clay mineral suites can be adequately described using "inert" chemical components, i.e., those which are extensive variables of the system, and by using pressure-temperature as intensive variables. Hydrothermal alteration is, in contrast, an environment where many chemical components can be treated as being completely... 

Layer-silicates Recent studies have also demonstrated the potential microbial influence on clay mineral (layer silicates) formation at hydrothermal vents. Bacterial cells covered (or completely replaced) with a Fe-rich silicate mineral (putative nontronite), in some cases oriented in extracellular polymers (as revealed by TEM analysis), were found in deep-sea sediments of Iheya Basin, Okinawa Trough (Ueshima Tazaki, 2001), and in soft sediments, and on mineral surfaces in low-temperature (2-50°C) waters near vents at Southern Explorer Ridge in the northeast Pacific (Fortin etal., 1998 Fig. 8.6). The Fe-silicate is believed to form as a result of the binding and concentration of soluble Si and Fe species to reactive sites (e.g. carboxyl, phosphoryl) on EPS (Ueshima Tazaki, 2001). Formation of Fe-silicate may also involve complex binding mechanisms, whereas metal ions such as Fe possibly bridge reactive sites within cell walls to silicate anions to initiate silicate nucleation (Fortin etal., 1998). Alt (1988) also reported the presence of nontronite associated with Mn- and Fe-oxide-rich deposits from seamounts on the EPR. The presence of bacteria-like filaments within one nontronite sample was taken to indicate that bacterial activity may have been associated with nontronite formation. Although the formation of clay minerals at deep-sea hydrothermal vents has not received much attention, it seems probable that based on these studies, biomineralisation of clay minerals is ubiquitous in these environments. 

While inheritance dominates in the sedimentary environment at generally ambient conditions characterized by slow reaction rates, layer transformation requires a considerable input of activation energy, and thus is found preferentially in the diagenetic and hydrothermal realms, where higher temperatures prevail. In between these two environments, the weathering environment exists in which all three mechanisms discussed above can be operational. Hence, when these three mechanisms occur in three different geologic environments, it leads to nine pos-sibiUties of clay mineral formation in nature, attesting to the exceptional variability and complexity of day mineral chemistries. 

These clays occur in limestones, dolomites, evaporites, shales, siltstones, and hydrothermal deposits. All the sedimentary material appears to have a diagenetic origin. Although the physical environments vary, the chemical environments should be similar. Saline or even super-saline conditions are implied by the presence of evaporite minerals associated with some of the deposits. In the other deposits it is possible that temporary evaporitic conditions (e.g., tidal flats) existed long enough for brucite to precipitate between the layers of expanded-layer minerals. It appears plausible that the parent material was a montmorillonite-like mineral (probably detrital in most cases). 

Chamosite appears to be the finest grained and most abundant mineral in this group. It occurs in lateritic clay deposits (Brindley, 1951), both as oolites and matrix in sedimentary ironstones (Hallimond, 1925), in hydrothermal deposits (Ruotsala et al., 1964), in shales (Drcnnan, 1963), in Recent shallow-marine deposits (Porrenga, 1966) and in estuarine sediments (Rohrlich et al., 1969). It is probable that chamosite is more abundant than commonly realized however, Drennan (1963) has pointed out that it is extremely unstable in a leached and oxidized environment and is not likely to persist as an allogenic mineral. 

However, amino acids are unlikely to form themselves into polymers without the help of some form of catalyst (Bada, 2004). Possible natural catalysts are mineral surfaces such as in the regular, repeating structure of clays although once bound to a clay the polymer has to be released. This is achieved in the laboratory with salt solutions and may, in nature, reflect an evaporative marine environment. An alternative venue is at hydrothermal vents where peptide bond formation is favored, and where catalysis may take place on sulfide mineral surfaces (Bada, 2004). Such a process has been described by Holm and Charlou (2001) who found linear saturated hydrocarbons with chain lengths of 16 to 29 carbon atoms in high-temperature hydrothermal fluids from a vent in the Mid-Atlantic Ridge. 

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