Chemical Thermodynamics of Silica: a Critique on Its Geothermometer (original) (raw)
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Silicate solid solutions and geothermometry
Contributions to Mineralogy and Petrology, 1969
The properties of the regular soh,tion arc described. A solid solution is assigned certain theoretical wt.hms of the heat of mixing and the results of the distribution of a component between two coexisting soluti<ms are shown graphically on I{oozcboom diagrams. Such rClU'esentati(ms may be (asefl0 in explaining the distribution of elements observed i1~ natural mineral assemt)lages.
2016
The theory of silica-water system or silica solubility is quite complex as it involves dissolution, polymerization and precipitation processes to form the silicate scale. The type and amount of silicate scale observed is dependent on several factors including pH, salinity, magnesium concentration, and the ratio of calcium to magnesium. This paper describes the impact of some factors including brine salinity and composition on silica solubility using crash quartz sandstone core samples. Synthetic brine with different salinities ranged between 20,000ppm to 60,000ppm were utilized to determine the change in soluble silica at 80°C. The amount of silica dissolved from the sandstone sample was measured using Silicomolybdate method and validated using Energy Dispersive X-RAY and X-RAY Diffraction techniques. The results clearly showed that the composition of the brine has a significant effect on the silica solubility. The amount of silica dissolution increased from 41.0mg/L to 67.8mg/L whe...
European Journal of Mineralogy, 2005
Recent systematic studies of mineral solubilities in water to high pressures up to 50 kbar call for a suitable thermodynamic formalism to allow realistic fitting of the experimental data and the establishment of an internally consistent data base. The very extensive low-pressure (< 5 kbar) experimental data set on the solubility of SiO 2 in H 2 O has in the last few years been extended to 20 kbar and 1300°C, providing an excellent experimental basis for testing new approaches. In addition, solubility experiments with different SiO 2-buffering phase assemblages and in situ determinations of Raman spectra for H 2 O-SiO 2 fluids have provided both qualitative and quantitative constraints on the stoichiometry and quantities of dissolved silica species. We propose a thermodynamic formalism for modeling both absolute silica solubility and speciation of dissolved silica using a combination of the chain reaction approach and a new Gibbs free energy equation of water based on a homogeneous reaction formalism. For a given SiO 2-buffer (e.g., quartz) and the coexisting H 2 O-SiO 2 fluid both solubility and speciation of silica can be described by the following two reactions:-monomer-forming standard reaction: SiO 2(s) + 2(H 2 O)L = (SiO 2)•(H 2 O) 2 (A)-polymer-forming chain reaction: (SiO 2)n-1 •(H 2 O) n + (SiO 2)•(H 2 O) 2 = (SiO 2) n •(H 2 O) n+1 + (H 2 O) L , (B) where 2 ≤ n ≤ ∞, and (H 2 O) L stands for "liquid-like" (associated, clustered) water molecules in the aqueous fluid. We show that reactions (A) and (B) lead to the simplified relationships ∆G°(mono),r,P,T = ∆H°(mono),r-T∆S°(mono),r + ∆Cp°(mono),r [T-298.15-Tln(T/298.15)] + ∆V°(mono),r (P-1), and ∆G°(poly),r,P,T = ∆H°(poly),r-T∆S°(poly),r + ∆V°(poly),r (P-1) (where the ∆G°r ,P,T , are the standard molar Gibbs free energy changes in reactions (A) and (B) as a function of pressure P and temperature T; the ∆H°r, ∆S°r, ∆Cp°r and ∆V°r are standard molar enthalpy, entropy, isobaric heat capacity and volume changes, respectively, in reactions (A) and (B) at reference temperature T o = 298.15 K and pressure P o = 1 bar) that provide excellent descriptions of the available H 2 O-SiO 2 data set in terms of both SiO 2 solubility and silica speciation. Discrepancies between directly determined solubility data and data obtained from in situ Raman spectra are ascribed to (i) possible experimental problems of equilibration and (ii) inherent difficulties of interpreting Raman spectra of dilute H 2 O-SiO 2 solutions. In agreement with recent findings, our model indicates that dissolved silica in quartz-buffered aqueous solutions is considerably polymerized, exceeding 20-25 % at all temperatures above 400°C.
Analytical aspects of silica in saline water — application to desalination of brackish waters
Desalination, 2001
In several areas like water desalination and geothermal applications, the occurrence of silica in the used waters is undesirable. Many studies show the complexity and the diversity of the topic. For a better understanding of some of its analytical aspects the determination of amorphous silica content of natural waters, was carried out by various analytical methods. Amorphous silica solubility, at different temperatures, pH and ionic strength was studied. The heat of silica dissolution and the activity coefficient of dissolved silica were determined. A microcomputer program with the LabView graphical language was designed to calculate the solubility and concentration factors that indicate the limits above which silica precipitation occurs. This practical tool was applied to a variety of aqueous salt solutions and to some brackish waters from South Tunisia, at different temperatures. Predicted solubility values, using deduced silica activity coefficient, are in good agreement with experimental results.
The silica heat flow interpretation technique: Assumptions and applications
Journal of Geophysical Research, 1980
We have previously established a linear relation between temperatures based on the silica content of groundwater and regional heat flow and used the relation to prepare a new heat flow map of the continental United States. We now examine the assumptions upon which the relation is based, the accuracy to which groundwater silica data can be used to estimate regional heat flow, and the limitation of the technique. By averaging silica geotemperatures and traditional heat flow values over 1 o x 1 o blocks, the linear regression is TSiO2 --mq + b, where m and b are constants determined to be 680 :!: 67øC m 2 W -• and 12.4 :!: 5.1øC. The physical significance of b is mean annual surface temperature, and the product of m times thermal conductivity reflects the minimum mean depth to which groundwaters may circulate. These values are not sufficiently different from our earlier values (m --670, b = 13.2) to justify using the newer values. To illustrate the application of the linear regression in predicting regional heat flow, data sets are presented from upstate New York, south central New Mexico, and Egypt. In each case, the predicted heat flow is tectonically reasonable and consistent with whatever traditional data are available. S( $7-•8) ß 120-S(37-120) ß ß ß ß . ß ß ß ! ß S(39-121) ß . . / ' I00--' ß ', ß ß ß . / -ß . I ß ß / . ß ß ß ee ee ß ß • ß ß e • ß ß -• ß ß ß e e ee ß ß 0 80-ß o . ß/ß ß ß _ o '. ). "" '.'.t •,.'/:' N ' ß ß e•' em ee ß ß eee ß ß ß • -o • 60 ß ß (4•-III) -I---' '. ß I t.y' : ':: ß •/; :' : . ß 40 ß ,4 '. .. ' R(37-104) //• ß . ' R(37-107) ß ß e•e•e ß ,,. ß ' :. '. ' ' ' R(39-106) ß ß / ' . . . ß. ' m = 59.6 t 16.2'C m2W -• -ß ß • ß ß . ee ß e e y e e . ß ß e e• ß ; ß ß . • ,,• . . NEW MEXICO ß ß ß ß ß ß SAN DIEGO MOUNTAIN DONA ANA CO. -ZONE AFFECTED BY THE RIO GRANDE ß RADIUM II ß SPRINGS i ß ß ß : ß ß ß ß ß LAS ALTURAS ß ß ß ß ß ß X KILBOuRNE HOLE IIII ß TEXAS ß ß ß NEW MEXICO MEXICO
Changes in silica chemistry and hydrology across the Rotorua geothermal field, New Zealand
Geothermics, 1998
A comprehensive review of field data from Rotorua geothermal field, New Zealand, was undertaken to evaluate the mixing relationships of cooler groundwaters with the primary fluid. Unlike previous studies the reservoir well compositions were calculated using only the most reliable measured downhole fluid temperatures. Inconsistencies in gas chemistry showed that the hot fluid in the southeast could not be derived from a postulated 250°C parent by simple boiling, a mechanism proposed by previous authors. Mixing relationships showed conclusively that across the field from southeast to northwest, all the fluids were related along a mixing line with low-chloride water at 150°C. Between 158 and 175°C the diluting fluid changed to a low-chloride groundwater at 15°C.An SiO2-temperature plot of all the well data showed that silica values for the hot wells fall around the quartz solubility line. The data for the majority of the cooler wells scatter about the α-cristobalite solubility line. Plots of the Cl/SiO2 ratio were used to evaluate changes in silica chemistry across the reservoir by comparing measured data with theoretical mixing lines, assuming partial equilibrium with respect to the three silica polymorphs, quartz, α-cristobalite and amorphous silica. The only assumptions made in the calculation were that the 150 and 15°C diluting fluids were in equilibrium with cristobalite and amorphous silica, respectively. Surprisingly, the trends may be explained by mixing with cooler silica-rich diluting fluids without requiring equilibration with respect to any of the silica polymorphs.
2014
Dieng Geothermal Field has been long encountered with a severe production issue regarding to silica scaling, in which has also been a serious problem in many other water-dominated hydrothermal fields. This paper examines silica scaling occurence in the surface facilities of Dieng Geothermal Field, e.g. separator, brine pipeline, and weirbox, and thus most likely has affected adversely to the production activities of the field. Accordingly, the deposition of silica is highly dependent on temperature and pH of fluid. In which, the estimation of silica deposition at two representative production wells, Well A and Well B, was obtained by sampling of separator water and calculating the SiO2 concentration within the brine. The value of silica deposition was obtained by calculating the difference between the value of dissolved SiO2 with the solubility of amorphous silica curve in respect of the temperature and pH. The result of this research shows that the process of silica scaling formati...
Solubility of fused silica in sub- and supercritical water: Estimation from a thermodynamic model
The Journal of Supercritical Fluids, 2013
Near-and supercritical water appears to be a useful green agent for surface treatment of fused silicabased devices to be used in separation methods of analytical chemistry. To assist the development of the devices, a simple model has been developed to estimate the aqueous solubility of fused (amorphous) silica as a function of temperature and pressure. The central assumption of the model is that, at the particular temperature and pressure, the aqueous species formed by dissolution of SiO 2 are the same regardless of whether they resulted from dissolution of quartz or from dissolution of fused silica. Conceptually, the model combines a correlation of the aqueous solubility of quartz with a thermochemical cycle to estimate the ratio of the fugacity of fused silica to the fugacity of quartz. The existing experimental data on aqueous solubility of fused (amorphous) silica are used to build up the model, and they are reproduced by the model with an average relative deviation of 2.9%. Depending on the temperature and pressure, the "enrichment factor" of the aqueous solubility of fused silica over that of quartz is predicted to range from 2 to 18.
Thermochemistry and kinetics of silica dissolution in NaOH aqueous solution
Thermochimica Acta, 2011
Dissolution of increasing amounts of silica (␣-cristobalite) in the same volume of a 4% weight of sodium hydroxide solution was followed by microcalorimetry in a closed cell at temperature up to 130 • C. The process led to the formation of a dissolved entity whose formula depends on the Na 2 O/SiO 2 ratio. At the beginning dissolution gave [3Na 2 O, 4SiO 2 ] entity and when increasing dissolved silica amounts, the latter transforms into [Na 2 O, 2SiO 2 ]. Kinetic study showed that dissolution starts by a very rapid step with about 10% of the whole heat energy released and continued with a very slow process. Kinetic analysis showed that the latter agrees with two successive silica first-order reactions scheme. The resulting activation energy values are of the same magnitude order of that deduced from the isoconversional model.