Silica Springs,Tongariro National Park,New Zealand—analyses of the spring water and characterisation of the alumino-silicate deposit |
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| Authors: | N Wells CW Childs CJ Downes |
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| Institution: | Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt, New Zealand;Chemistry Division, Department of Scientific and Industrial Research, Petone, New Zealand |
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| Abstract: | The creamy-white deposit in the stream bed below Silica Springs outlet on Mount Ruapehu, Tongariro National Park, New Zealand, has been identified as a hydrous, X-ray-amorphous, aluminosilicate (allophane). The SiO2/Al2O3 mole ratio varies from close to one, to close to two. The elements K, Ca, Mn and Fe are present in low concentrations relative to those in allophanic soil clays, and tend to increase in concentration downstream from where the deposit first occurs. The concentration of S decreases downstream from 0.5% to 0.1%. Surface areas of samples, measured by the ethylene glycol desorption method, are about 200–300 m2/g. The outlet water at Silica Springs contains low dissolved solids and is undersaturated with respect to amorphous silica, but is supersaturated with respect to several alumino-silicate minerals (of which allophane may be considered the precursor) and with respect to CO2. Gas bubbling at the outlet contains about 10% CO2 which has a δC13PDB value of ?7.5%..Silica Springs water is derived from the addition of geothermal CO2 (and possibly H2S) to near-surface meteoric water from the lava flow above the outlet, and the chemical attack of this water on the andesitic rocks and soil through which it passes. The pH of water at Silica Springs increases from 5.45 at the outlet, to 5.90 where deposition first occurs, to 6.80 below the region of maximum deposition. This rise in pH correlates with loss of excess CO2 in turbulent regions of the stream, and, through surface charge effects, is probably an important influence on the site of deposition, which begins approx. 100m downstream from the outlet. |
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