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Scientific paper
Mar 1970
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1970gecoa..34..295h&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 34, Issue 3, pp.295-300
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4
Scientific paper
Until now quartz has only been synthesized under hydrothermal conditions. On the basis of the experimental data of (1964) and (1966) it had been assumed that at temperatures below 100°C the formation of quartz from amorphous silica would take a long time, possibly thousands of years. The solubility of amorphous silica in water at 25°C and at a pH below 9 is about 120 ppm SiO 2 , with silica in solution in the monomeric form. It has been shown that even from strongly undersaturated solutions (concentrations down to 0.5 ppm SiO 2 ) silica can be adsorbed by hydroxides of iron, aluminum, manganese, magnesium and other elements. The SiO 2 concentration of such solutions is comparable to those of natural waters. Through this adsorption process silica is enriched in the precipitates compared with the corresponding solution. The exact behaviour of the different hydroxides in these adsorption processes will be discussed in future publications. At this time it suffices to say that such X-ray amorphous iron-, aluminum-, manganese-, and magnesium-hydroxide-silica-precipitates can contain as much as 90 mole percent SiO 2 . Earlier ( , 1965) it was suggested that during diagenesis quartz could be formed from such precipitates. (1967) has shown that in the temperature range of 0°-80°C quartz can form in a very short time from such X-ray amorphous hydroxide-silica-precipitates by aging in silica solutions. Under suitable conditions quarz was detected after 14 days. The formation of quartz was observed in hydroxide-silica-precipitates of iron (III) as well as in those of aluminum, manganese, magnesium and other elements. In this paper results of experiments with iron (III) hydroxides are presented. The newly formed quartz was identified by optical and X-ray methods. The optical investigations revealed that well-shaped idiomorphic quartz crystals were scarce and their average grain size was between 0.01 mm and 0.1 mm. Frequently these quartz crystals contained inclusions of hydroxides. Along with the quartz, quartzin is always formed (quartzin = quartz elongated parallel to the z -axis). Besides quartz and quartzin, amorphous silica is present in the precipitates in variable amounts depending upon the experimental conditions. The experiments so-far completed show that the formation of quartz in hydroxide-silica precipitates is influenced by the following factors: 1. (a) SiO 2 concentration in the solution Formation of quartz takes place to a larger extent only in those precipitates which are in reaction with solutions undersaturated with respect to amorphous silica. If the SiO 2 concentration in the solution lies above the solubility of amorphous silica, no appreciable formation of quartz occurs in the precipitates. It seems that polymerisation of the silicic acid inhibits this process. The only crystalline compound formed under these conditions initially is quartzin. In highly supersaturated solutions the silica in the precipitates remains amorphous even after long periods of aging. 2. (b) Temperature The solubility of SiO 2 as well as the adsorption behaviour (of the hydroxides) are dependent on temperature. Consequently the formation of quartz is also temperature dependent, i.e. at higher temperatures the region of quartz formation is shifted towards higher SiO 2 concentrations in the solution (see table). Since the conditions of the experiments are comparable to those in natural sedimentary environments, one can expect that in these environments quartz is formed by a similar process. In natural settings, the hydroxides of iron, aluminum, manganese, magnesium, etc. adsorb silica and thus enable the formation of quartz in marine and fresh water environments. In recent iron-containing CO 2 -springs such quartz formations can be studied in some detail. Geochemical considerations indicate that the silica released during weathering of alkali feldspars will not be used up completely by the formation of clay minerals, since in the most abundant clay minerals the Si: Al ratios are below 2:1 whereas in alkali feldspars the ratio is 3:1. During alteration of alkali feldspars to clay minerals silica is left over and from this excess silica, quartz can be formed. Geochemical balance calculations reveal that a considerable portion of the total amount of quartz in sediments is formed in the sedimentary environment and does not originate from magmatic processes.
Flehmig W.
Harder Hartwig
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