LECTURES ON GEOCHEMICAL !N1 ERPRETATION OF ...

- 21 -THE SOLUBILITY OF SILICA IN HYDROTHERMAL SOLUTIONS:PRACTICAL APPLICATIONSRobert O. Fou~ierU.s. Geological Survey. Menla Park. california 94025 U.S.A.INTRODUCTIONThe soIubilities of the most common silica minerals have been determinedexperimentally as functions of temperature at the vapor pressure ofthe solution. Pressure and added salt have little effect on the solubilitiesof quartz and amorphous silica below about 300°C. Above 300°C, bothpressure and added salt are very important. This information allows thedissolved silica coneentration in a hydrothe~l solution to be used as achemical geothermometer. When using the silica geothermometer, an assumptionmust be made about the particular silica mineral that is cont~ollingdissolved silica. and co~~ections may be ~equi~ed fo~ the effects of decomp~essionalboiling (adiabatic cooling). Also. below 340·C the solubilityof silica dec~eases d~astically as tempe~atu~e decreases. so silica mayprecipitate from solution as a result of conductive or adiabatic coolingbefore ~eachingthe surface. causing low estimated reservoir tempe~atures.In addition to causing problems in the application of the silica geothermometer.the precipitation of silica (sealing) can be a major problemduring production and disposal of geothermal fluids. The~efore. theprediction of conditions at which silica polymerization and scaling willoccur is of great importance in the design and ope~ationinstallations.of geothe~alSILICA GEOTHERKOMETRYThe silica geothe~ometer of Fou~ier and Rowe (1966) was p~esentedgraphically as two curves that could be used in the range 120° to 330°Cwith the following assumptions : the fluid was in equilibrium with quartzin the reservoir. the po~e - fluid prcssu~e in the reservoir is fixed by the