Formation water olivine

In the low-temperature region pyroxene and olivine can be retained only in dry rocks containing no volatiles. In the presence of a fluid phase the appearance of cummingtonite should be expected in the case of excess water, or carbonates in the case of excess carbon dioxide, which happens in hydrothermal metasomatism or even in progressive metamorphism of silicate or carbonate iron formations, respectively. 

The association pyroxene + olivine + cummingtonite + quartz, often encountered in highly metamorphosed BIF, is also very important for judging the thermodynamic parameters of mineralization. This association fixes the temperature at 700-720°C, which depends little on pressure and iron content of the silicates, while pressure can be estimated fairly precisely from the iron content of orthopyroxene. A decrease in partial pressure of water in conjunction with mechanical equilibrium P =Ff) due to dilution of the fluid by other volatiles, for instance carbon dioxide, can lead to some shift of the P-T curve into the lower-temperature region. However, the amount of such a shift, especially at high pressures (8-10 kbar) cannot be significant because Fe-Mg carbonates which are stable in carbon dioxide fluids already appear at 610-650°C (Fig. 93b), and their paragenesis with anhydrous silicates is not typical of iron formations. 

When olivine and pyroxene are oxidized the same regularities are observed as in the oxidation of carbonates. In particular, it was established in a study of the rich and unique olivine-magnetite iron ores of the Volodarsk deposit (Ukrainian shield) that the magnesium content of the silicates increases as the magnetite content of the ore increases (Mel nik and Yaroshchuk, 1966). In this case the most likely oxidants may be water and carbon dioxide, the main components of the fluids causing metasomatic reworking of the olivine-and pyroxene-bearing iron formations. 

This can be illustrated by a natural example. In the coarse-grained Allanin magnesium-gabbro, infiltration of fluid caused the formation of reaction rims around olivine (Chinner and Dixon, 1973). The succession is olivine anthophyllite (2 wt.% H2O) — talc (4wt.% H2O + kyanite — chloritoid (8wt.% H20- -talc + kyanite. This reaction rim is H2O undersaturated, and the succession of mineral assemblages corresponds to an increase of H2O content towards the rim and can only be modeled by an increase in the availability of water towards the rim. The H20-undersamrated character of the inner rim zones does not necessitate (or justify) a CO2 component in the fluid, but rather reflects limited availability of an H2O fluid. 

Asbestos, the first inorganic fiber material used, is currently still exclusively produced from natural mineral deposits. It is formed by the hydrothermal conversion of basic and ultrabasic volcanic rock (olivine and pyroxene) to serpentine upon which the actual asbestos formation takes place leading to two asbestos sorts with different structures serpentine asbestos and amphibole asbestos. Asbestos can be produced synthetically by several hours heating of a polysilicic acid/metal oxide mixture (e.g. Mg, Fe, Co, Ni) in water at 300 to 350°C and 90 to 160 bar. The properties of four important asbestos types are summarized in Table 5.2-2. 

Most geologic formations are comprised of variable amounts of minerals that are thermodynamically stable at low temperatures, such as quartz, calcite, and ferric oxyhydroxides, along with minerals that formed at high temperatures, such as the olivines and pyroxenes and calcic feldspars. These high-temperature phases are thermodynamically unstable in most low-temperature weathering environments and so rarely equilibrate with the water. (See Chap. 7.) Groundwaters in such systems may equilibrate with low-temperature secondary minerals that have formed by breakdown of the high-temperature phases. 

The mechanism of forming H2 on grain mimics has received a good deal of attention over the last few years. Vidali and co-workers [93, 94] studied the formation of HD after exposing a number of surfaces (olivine, amorphous carbcm and water ice at 5-20 K) to beams of H and D generated in microwave discharges. 

Some similarities with the Murchison meteorite can be noticed. The Murchison mineral structure is dominated with a phyllosilicate (serpentine) matrix which contains minerals such as olivine, pyroxenes, calcium carbonates, iron oxides (magnetite), iron-nickel sulfides and sulfates [23-25]. It has been altered by water, by heat, by pressure shock waves, by short-lived radionuclides [26,27]. The transformation of olivine and pyroxene chondrules seems to grow with the extent of mineral hydrolysis and the formation of water-soluble organic compounds is described at temperatures below -125 C [28,29]. Aside from any terrestrial contamination, all the classes of organic molecules considered of biological relevance are identified [30-32 and Ref therein] and also non-terrestrial amino acids and enantiomeric excesses [33-35]. 

Ashworth, and Hutchison, 1975 [11] made electron microscopic observations of the hydrous alteration products of olivine in an achondrite and in an ordinary chondrite. Their conclusion was that the Nakhla achondrite, and possibly the Weston chondrite, contain water of extraterrestrial origin which was mobilized by mild shock deformation. Carbonaceous chondrites are believed to be unaltered material left over from the formation of the solar system. They contain substantial amounts of reduced carbon and of water in the form of hydroxyl ions. The oxidation state of iron in some carbonaceous chondrites has been determined by means of Moess-bauer spectroscopy, and it is demonstrated that there is a correlation between the oxidation state of iron and the content of water and reduced carbon in the meteorites (Roy-Poulsen et al., 1981 [284]). 

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