Chalcedony

Abstract: Length-slow chalcedony, having its c-axis parallel to the fibers, is unusual in nature. Occurrences so far investigated reveal that this rare type of optically fibrous silica occurs almost exclusively in association with sulphates and evaporites. Often the evaporite minerals have been totally removed, and thus the survival of resistant length-slow chalcedony reveals the presence of former salt-flat, sabkha or sulphate-rich environments where none were before suspected. This type of chalcedony also in some instances forms in semi-arid, alkaline soils. Discovery of this simple criterion has led to development of an additional series of characteristics for identification of silicified evaporites. All siliceous deposits should be re-examined for presence of this important genetic m rker.

Abstract: The calcretes of the Kalahari are amongst the thickest in the world representing pedogenic episodes in a semi-arid climate during Pliocene to Recent times. The descriptive morphological terminology of Netterberg is used to describe the calcrete types and a differentiation into simple and composite profiles is made. A pedogenic/diagenetic scheme has been constructed using all available data. Early calcite cementation is induced by two mechanisms. Slow evaporation and/or CO2 loss causes the precipitation of low-Mg calcite, whereas rapid evaporation and/or CO2 loss precipitates predominantly high-Mg calcite, in thermodynamic disequilibrium with the low Mg/Ca ratio vadose water, and minor aragonite (which transforms rapidly to low-Mg calcite). High-Mg calcite is also precipitated from high Mg/Ca ratio vadose waters in calcretes developed on Mg-rich host lithologies and by capillary rise from shallow groundwaters in saline depressions. Calcite precipitation may be passive (cement), displacive or replacive, in the latter released silica migrating down-profile to precipitate length-slow chalcedony, clinoptilolite (saline conditions), length-fast chalcedony and megaquartz (non-saline conditions). Displacive introduction of calcite takes place from highly supersaturated solutions due to rapid evaporation (with CO2 loss) of vadose waters. During low-Mg calcite precipitation (in a ‘closed’ system) the Mg concentration of the resulting solution increases. This, combined with Mg released during high-Mg to low-Mg calcite transformation, induces precipitation of authigenic palygorskite, sepiolite and minor dolomite. Vadose dolomite is often present whilst some dolomite may be precipitated at the mixing-zone of vadose and phreatic waters. The proposed model applies to the Kalahari calcretes, although it may perhaps be extrapolated to other areas. Further detailed studies, involving analyses of pore water chemistry, soil microclimate, and trace element and isotopic analyses of individual cements are necessary.

Abstract: Temperatures measured in drillholes indicate that the amount of dissolved SiO/sub 2/ in thermal waters at depth in the low temperature hydrothermal areas in Iceland is governed by the solubility of chalcedony at below approximately 110/sup 0/. At above approximately 180/sup 0/, the solubility of quartz governs the amount of dissolved SiO/sub 2/ in the water. The factors that determine with which of these SiO/sub 2/ minerals the dissolved SiO/sub 2/ may reach equilibrium in this temperature range were not shown. Equilibrium with chalcedony appears in some instances to be established at 20/sup 0/, but most commonly cold ground water is SiO/sub 2/ supersaturated with respect to chalcedony solubility. Provided SiO/sub 2/ is the only varying component in the system, SiO/sub 2/ temperatures are minimum temperatures and represent temperatures of last equilibrium with quartz or chalcedony. If mixing with cold ground water takes place and an originally high pH of the thermal water is lowered, the estimated SiO/sub 2/ temperatures are likely to be high compared with the temperature of the mixture. There is a clear relation between the intensity of low temperature hydrothermal activity in Iceland and the age of the rocks in which this activity is located.