Decrease in the existing experimental silica solubility data along the water-vapor saturation curve after 300?C is due to analytical error associated with neglecting the effect of vapor proportion in the reaction vessel. This effect is very pronounced at high temperatures as the specific volume of water and vapor are of same order of magnitude. Therefore the existing solubility data could be considered as reliable only up to 300?C. The respective regression expressions for quartz and amorphous silica are


A combined evaluation of quartz solubility data along the water-vapor saturation and in the compressed liquid region together with the PVT characteristics of water suggests that these regression expressions are valid of whole range of temperature from 0 to 374?C.
It is concluded that the fundamental limitation to use these equations as geothermometers is the correction of total discharge silica composition from a well for the vapor fraction in the geothermal reservoir. For knowing the vapor fraction there is indirectly need of knowing the reservoir temperature. Therefore, the use of these equations is more likely to know the state of silica-water interaction in the reservoir.

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